Patchwork [v2,1/8] target-arm: A64: add support for ld/st pair

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Submitter Peter Maydell
Date Dec. 11, 2013, 10:01 p.m.
Message ID <1386799315-4431-2-git-send-email-peter.maydell@linaro.org>
Download mbox | patch
Permalink /patch/300464/
State New
Headers show

Comments

Peter Maydell - Dec. 11, 2013, 10:01 p.m.
From: Alex Bennée <alex.bennee@linaro.org>

This patch support the basic load and store pair instructions and
includes the generic helper functions:

  * do_gpr_st()
  * do_fp_st()
  * do_gpr_ld()
  * do_fp_ld()
  * read_cpu_reg_sp()
  * gen_check_sp_alignment()

The last function gen_check_sp_alignment() is a NULL op currently but
put in place to make it easy to add SP alignment checking later.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

---

Changes
  - merge ldp/stp together
  - add read_cpu_reg_sp() and use
  - only reference registers as needed
  - use common tcg ops for gpr/fp loads/store
---
 target-arm/translate-a64.c | 268 ++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 263 insertions(+), 5 deletions(-)
Claudio Fontana - Dec. 12, 2013, 9:54 a.m.
Hi,

I saw a missing return below:

On 11 December 2013 23:01, Peter Maydell <peter.maydell@linaro.org> wrote:
> From: Alex Bennée <alex.bennee@linaro.org>
>
> This patch support the basic load and store pair instructions and
> includes the generic helper functions:
>
>   * do_gpr_st()
>   * do_fp_st()
>   * do_gpr_ld()
>   * do_fp_ld()
>   * read_cpu_reg_sp()
>   * gen_check_sp_alignment()
>
> The last function gen_check_sp_alignment() is a NULL op currently but
> put in place to make it easy to add SP alignment checking later.
>
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
>
> ---
>
> Changes
>   - merge ldp/stp together
>   - add read_cpu_reg_sp() and use
>   - only reference registers as needed
>   - use common tcg ops for gpr/fp loads/store
> ---
>  target-arm/translate-a64.c | 268 ++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 263 insertions(+), 5 deletions(-)
>
> diff --git a/target-arm/translate-a64.c b/target-arm/translate-a64.c
> index 0a76130..018b3ee 100644
> --- a/target-arm/translate-a64.c
> +++ b/target-arm/translate-a64.c
> @@ -99,6 +99,15 @@ void aarch64_cpu_dump_state(CPUState *cs, FILE *f,
>      cpu_fprintf(f, "\n");
>  }
>
> +static int get_mem_index(DisasContext *s)
> +{
> +#ifdef CONFIG_USER_ONLY
> +    return 1;
> +#else
> +    return s->user;
> +#endif
> +}
> +
>  void gen_a64_set_pc_im(uint64_t val)
>  {
>      tcg_gen_movi_i64(cpu_pc, val);
> @@ -250,6 +259,17 @@ static TCGv_i64 read_cpu_reg(DisasContext *s, int reg, int sf)
>      return v;
>  }
>
> +static TCGv_i64 read_cpu_reg_sp(DisasContext *s, int reg, int sf)
> +{
> +    TCGv_i64 v = new_tmp_a64(s);
> +    if (sf) {
> +        tcg_gen_mov_i64(v, cpu_X[reg]);
> +    } else {
> +        tcg_gen_ext32u_i64(v, cpu_X[reg]);
> +    }
> +    return v;
> +}
> +
>  /* Set ZF and NF based on a 64 bit result. This is alas fiddlier
>   * than the 32 bit equivalent.
>   */
> @@ -278,6 +298,124 @@ static inline void gen_logic_CC(int sf, TCGv_i64 result)
>  }
>
>  /*
> + * Load/Store generators
> + */
> +
> +/*
> + *  Store from GPR register to memory
> + */
> +static void do_gpr_st(DisasContext *s, TCGv_i64 source,
> +                      TCGv_i64 tcg_addr, int size)
> +{
> +    g_assert(size <= 3);
> +    tcg_gen_qemu_st_i64(source, tcg_addr, get_mem_index(s), MO_TE + size);
> +}
> +
> +/*
> + * Load from memory to GPR register
> + */
> +static void do_gpr_ld(DisasContext *s, TCGv_i64 dest, TCGv_i64 tcg_addr,
> +                      int size, bool is_signed, bool extend)
> +{
> +    TCGMemOp memop =  MO_TE + size;
> +
> +    g_assert(size <= 3);
> +
> +    if (is_signed) {
> +        memop += MO_SIGN;
> +    }
> +
> +    tcg_gen_qemu_ld_i64(dest, tcg_addr, get_mem_index(s), memop);
> +
> +    if (extend && is_signed) {
> +        g_assert(size < 3);
> +        tcg_gen_ext32u_i64(dest, dest);
> +    }
> +}
> +
> +/*
> + * Store from FP register to memory
> + */
> +static void do_fp_st(DisasContext *s, int srcidx, TCGv_i64 tcg_addr, int size)
> +{
> +    /* This writes the bottom N bits of a 128 bit wide vector to memory */
> +    int freg_offs = offsetof(CPUARMState, vfp.regs[srcidx * 2]);
> +    TCGv_i64 tmp = tcg_temp_new_i64();
> +
> +    if (size < 4) {
> +        switch (size) {
> +        case 0:
> +            tcg_gen_ld8u_i64(tmp, cpu_env, freg_offs);
> +            break;
> +        case 1:
> +            tcg_gen_ld16u_i64(tmp, cpu_env, freg_offs);
> +            break;
> +        case 2:
> +            tcg_gen_ld32u_i64(tmp, cpu_env, freg_offs);
> +            break;
> +        case 3:
> +            tcg_gen_ld_i64(tmp, cpu_env, freg_offs);
> +            break;
> +        }
> +        tcg_gen_qemu_st_i64(tmp, tcg_addr, get_mem_index(s), MO_TE + size);
> +    } else {
> +        TCGv_i64 tcg_hiaddr = tcg_temp_new_i64();
> +        tcg_gen_ld_i64(tmp, cpu_env, freg_offs);
> +        tcg_gen_qemu_st_i64(tmp, tcg_addr, get_mem_index(s), MO_TEQ);
> +        tcg_gen_qemu_st64(tmp, tcg_addr, get_mem_index(s));
> +        tcg_gen_ld_i64(tmp, cpu_env, freg_offs + sizeof(float64));
> +        tcg_gen_addi_i64(tcg_hiaddr, tcg_addr, 8);
> +        tcg_gen_qemu_st_i64(tmp, tcg_hiaddr, get_mem_index(s), MO_TEQ);
> +        tcg_temp_free_i64(tcg_hiaddr);
> +    }
> +
> +    tcg_temp_free_i64(tmp);
> +}
> +
> +/* Load from memory to FP register */
> +static void do_fp_ld(DisasContext *s, int destidx, TCGv_i64 tcg_addr, int size)
> +{
> +    /* This always zero-extends and writes to a full 128 bit wide vector */
> +    int freg_offs = offsetof(CPUARMState, vfp.regs[destidx * 2]);
> +    TCGv_i64 tmplo = tcg_temp_new_i64();
> +    TCGv_i64 tmphi;
> +
> +    if (size < 4) {
> +        TCGMemOp memop =  MO_TE + size;
> +        tmphi = tcg_const_i64(0);
> +        tcg_gen_qemu_ld_i64(tmplo, tcg_addr, get_mem_index(s), memop);
> +    } else {
> +        TCGv_i64 tcg_hiaddr;
> +        tmphi = tcg_temp_new_i64();
> +        tcg_hiaddr = tcg_temp_new_i64();
> +
> +        tcg_gen_qemu_ld_i64(tmplo, tcg_addr, get_mem_index(s), MO_TEQ);
> +        tcg_gen_addi_i64(tcg_hiaddr, tcg_addr, 8);
> +        tcg_gen_qemu_ld_i64(tmphi, tcg_hiaddr, get_mem_index(s), MO_TEQ);
> +        tcg_temp_free_i64(tcg_hiaddr);
> +    }
> +
> +    tcg_gen_st_i64(tmplo, cpu_env, freg_offs);
> +    tcg_gen_st_i64(tmphi, cpu_env, freg_offs + sizeof(float64));
> +
> +    tcg_temp_free_i64(tmplo);
> +    tcg_temp_free_i64(tmphi);
> +}
> +
> +static inline void gen_check_sp_alignment(DisasContext *s)
> +{
> +    /* The AArch64 architecture mandates that (if enabled via PSTATE
> +     * or SCTLR bits) there is a check that SP is 16-aligned on every
> +     * SP-relative load or store (with an exception generated if it is not).
> +     * In line with general QEMU practice regarding misaligned accesses,
> +     * we omit these checks for the sake of guest program performance.
> +     * This function is provided as a hook so we can more easily add these
> +     * checks in future (possibly as a "favour catching guest program bugs
> +     * over speed" user selectable option).
> +     */
> +}
> +
> +/*
>   * the instruction disassembly implemented here matches
>   * the instruction encoding classifications in chapter 3 (C3)
>   * of the ARM Architecture Reference Manual (DDI0487A_a)
> @@ -620,10 +758,130 @@ static void disas_ld_lit(DisasContext *s, uint32_t insn)
>      unsupported_encoding(s, insn);
>  }
>
> -/* Load/store pair (all forms) */
> -static void disas_ldst_pair(DisasContext *s, uint32_t insn)
> -{
> -    unsupported_encoding(s, insn);
> +/*
> + * C5.6.81 LDP (Load Pair - non vector)
> + * C5.6.82 LDPSW (Load Pair Signed Word - non vector)
> + * C5.6.177 STP (Store Pair - non vector)
> + * C6.3.165 LDP (Load Pair of SIMD&FP)
> + * C6.3.284 STP (Store Pair of SIMD&FP)
> + *
> + *  31 30 29   27  26 25   23  22 21   15 14   10 9    5 4    0
> + * +-----+-------+---+-------+---+-----------------------------+
> + * | opc | 1 0 1 | V | index | L |  imm7 |  Rt2  |  Rn  | Rt   |
> + * +-----+-------+---+-------+---+-------+-------+------+------+
> + *
> + * opc: LDP&STP       00 -> 32 bit, 10 -> 64 bit
> + *      LDPSW         01
> + *      LDP&STP(SIMD) 00 -> 32 bit, 01 -> 64 bit, 10 -> 128 bit
> + *   V: 0 -> GPR, 1 -> Vector
> + * idx: 001 -> post-index, 011 -> pre-index, 010 -> signed off
> + *   L: 0 -> Store 1 -> Load
> + *
> + * Rt, Rt2 = GPR or SIMD registers to be stored
> + * Rn = general purpose register containing address
> + * imm7 = signed offset (multiple of 4 or 8 depending on size)
> + */
> +static void handle_ldst_pair(DisasContext *s, uint32_t insn)
> +{
> +    int rt = extract32(insn, 0, 5);
> +    int rn = extract32(insn, 5, 5);
> +    int rt2 = extract32(insn, 10, 5);
> +    int64_t offset = sextract32(insn, 15, 7);
> +    int index = extract32(insn, 23, 2);
> +    bool is_vector = extract32(insn, 26, 1);
> +    bool is_load = extract32(insn, 22, 1);
> +    int opc = extract32(insn, 30, 2);
> +
> +    bool is_signed = false;
> +    bool postindex = false;
> +    bool wback = false;
> +
> +    TCGv_i64 tcg_addr; /* calculated address */
> +    int size;
> +
> +    if (is_vector) {
> +        if (opc == 3) {
> +            unallocated_encoding(s);
> +            return;
> +        }
> +        size = 2 + opc;
> +    } else {
> +        size = 2 + extract32(opc, 1, 1);
> +        if (is_load) {
> +            is_signed = opc & 1;
> +        } else if (opc & 1) {
> +            unallocated_encoding(s);
> +            return;
> +        }
> +    }
> +
> +    switch (index) {
> +    case 1: /* post-index */
> +        postindex = true;
> +        wback = true;
> +        break;
> +    case 2: /* signed offset, rn not updated */
> +        postindex = false;
> +        break;
> +    case 3: /* pre-index */
> +        postindex = false;
> +        wback = true;
> +        break;
> +    default: /* Failed decoder tree? */
> +        unallocated_encoding(s);
> +        break;
> +    }

This doesn't seem right (break instead of return):

default:
    unallocated_encoding(s);
    return;
}

> +
> +    offset <<= size;
> +
> +    if (rn == 31) {
> +        gen_check_sp_alignment(s);
> +    }
> +
> +    tcg_addr = read_cpu_reg_sp(s, rn, 1);
> +
> +    if (!postindex) {
> +        tcg_gen_addi_i64(tcg_addr, tcg_addr, offset);
> +    }
> +
> +    if (is_vector) {
> +        if (is_load) {
> +            do_fp_ld(s, rt, tcg_addr, size);
> +        } else {
> +            do_fp_st(s, rt, tcg_addr, size);
> +        }
> +    } else {
> +        TCGv_i64 tcg_rt = cpu_reg(s, rt);
> +        if (is_load) {
> +            do_gpr_ld(s, tcg_rt, tcg_addr, size, is_signed, false);
> +        } else {
> +            do_gpr_st(s, tcg_rt, tcg_addr, size);
> +        }
> +    }
> +    tcg_gen_addi_i64(tcg_addr, tcg_addr, 1 << size);
> +    if (is_vector) {
> +        if (is_load) {
> +            do_fp_ld(s, rt2, tcg_addr, size);
> +        } else {
> +            do_fp_st(s, rt2, tcg_addr, size);
> +        }
> +    } else {
> +        TCGv_i64 tcg_rt2 = cpu_reg(s, rt2);
> +        if (is_load) {
> +            do_gpr_ld(s, tcg_rt2, tcg_addr, size, is_signed, false);
> +        } else {
> +            do_gpr_st(s, tcg_rt2, tcg_addr, size);
> +        }
> +    }
> +
> +    if (wback) {
> +        if (postindex) {
> +            tcg_gen_addi_i64(tcg_addr, tcg_addr, offset - (1 << size));
> +        } else {
> +            tcg_gen_subi_i64(tcg_addr, tcg_addr, 1 << size);
> +        }
> +        tcg_gen_mov_i64(cpu_reg_sp(s, rn), tcg_addr);
> +    }
>  }
>
>  /* Load/store register (all forms) */
> @@ -656,7 +914,7 @@ static void disas_ldst(DisasContext *s, uint32_t insn)
>          break;
>      case 0x28: case 0x29:
>      case 0x2c: case 0x2d: /* Load/store pair (all forms) */
> -        disas_ldst_pair(s, insn);
> +        handle_ldst_pair(s, insn);
>          break;
>      case 0x38: case 0x39:
>      case 0x3c: case 0x3d: /* Load/store register (all forms) */
> --
> 1.8.5
>
alex.bennee@linaro.org - Dec. 12, 2013, 11:43 a.m.
claudio.fontana@linaro.org writes:

> Hi,
>
> I saw a missing return below:
<snip>
>> +    default: /* Failed decoder tree? */
>> +        unallocated_encoding(s);
>> +        break;
>> +    }
>
> This doesn't seem right (break instead of return):
>
> default:
>     unallocated_encoding(s);
>     return;
> }

Good catch. I suspect it never hits though (or risu doesn't generate
enough unallocated versions).
Peter Maydell - Dec. 12, 2013, 11:45 a.m.
On 12 December 2013 11:43, Alex Bennée <alex.bennee@linaro.org> wrote:
>
> claudio.fontana@linaro.org writes:
>
>> Hi,
>>
>> I saw a missing return below:
> <snip>
>>> +    default: /* Failed decoder tree? */
>>> +        unallocated_encoding(s);
>>> +        break;
>>> +    }
>>
>> This doesn't seem right (break instead of return):
>>
>> default:
>>     unallocated_encoding(s);
>>     return;
>> }
>
> Good catch. I suspect it never hits though (or risu doesn't generate
> enough unallocated versions).

Oh, I saw that and then forgot to mention it.
Either the comment or the code is wrong -- if
the decoder tree prevents us getting to this point
with that value of index then we should be asserting,
not calling unallocated_encoding(). If the encoding
is really supposed to be unallocated then it's not
a failure of the decoder tree that we got here,
it's just another case we need to check. Which is it?

thanks
-- PMM
Claudio Fontana - Dec. 12, 2013, 12:14 p.m.
On 12 December 2013 12:45, Peter Maydell <peter.maydell@linaro.org> wrote:
> On 12 December 2013 11:43, Alex Bennée <alex.bennee@linaro.org> wrote:
>>
>> claudio.fontana@linaro.org writes:
>>
>>> Hi,
>>>
>>> I saw a missing return below:
>> <snip>
>>>> +    default: /* Failed decoder tree? */
>>>> +        unallocated_encoding(s);
>>>> +        break;
>>>> +    }
>>>
>>> This doesn't seem right (break instead of return):
>>>
>>> default:
>>>     unallocated_encoding(s);
>>>     return;
>>> }
>>
>> Good catch. I suspect it never hits though (or risu doesn't generate
>> enough unallocated versions).
>
> Oh, I saw that and then forgot to mention it.
> Either the comment or the code is wrong -- if
> the decoder tree prevents us getting to this point
> with that value of index then we should be asserting,
> not calling unallocated_encoding(). If the encoding
> is really supposed to be unallocated then it's not
> a failure of the decoder tree that we got here,
> it's just another case we need to check. Which is it?
>
> thanks
> -- PMM

I think that there is more than the missing return:

we need to handle the case 0 as well, as it's a perfectly valid form
of a load/store pair:
it's the Load/Store no-allocate pair (offset) (LDNP, STNP).

So in my view we need to add a case 0 where we handle  the load/store
no-allocate pair,
and no default case, or a default case where we assert(0) for unreachable code,
since all possible index values (2 bits) should be handled by the
switch statement.

Ref: C3.3 Loads and stores Table C3-3

Claudio
Peter Maydell - Dec. 13, 2013, 2:34 p.m.
On 12 December 2013 12:14, C Fontana <claudio.fontana@linaro.org> wrote:
> I think that there is more than the missing return:
>
> we need to handle the case 0 as well, as it's a perfectly valid form
> of a load/store pair:
> it's the Load/Store no-allocate pair (offset) (LDNP, STNP).
>
> So in my view we need to add a case 0 where we handle  the load/store
> no-allocate pair,
> and no default case, or a default case where we assert(0) for unreachable code,
> since all possible index values (2 bits) should be handled by the
> switch statement.

Yep. I've added support for the non-temporal versions
(pretty easy since qemu doesn't care about cache allocation
hints) to my patchstack so we'll get this in the next
version of the patchset.

thanks
-- PMM

Patch

diff --git a/target-arm/translate-a64.c b/target-arm/translate-a64.c
index 0a76130..018b3ee 100644
--- a/target-arm/translate-a64.c
+++ b/target-arm/translate-a64.c
@@ -99,6 +99,15 @@  void aarch64_cpu_dump_state(CPUState *cs, FILE *f,
     cpu_fprintf(f, "\n");
 }
 
+static int get_mem_index(DisasContext *s)
+{
+#ifdef CONFIG_USER_ONLY
+    return 1;
+#else
+    return s->user;
+#endif
+}
+
 void gen_a64_set_pc_im(uint64_t val)
 {
     tcg_gen_movi_i64(cpu_pc, val);
@@ -250,6 +259,17 @@  static TCGv_i64 read_cpu_reg(DisasContext *s, int reg, int sf)
     return v;
 }
 
+static TCGv_i64 read_cpu_reg_sp(DisasContext *s, int reg, int sf)
+{
+    TCGv_i64 v = new_tmp_a64(s);
+    if (sf) {
+        tcg_gen_mov_i64(v, cpu_X[reg]);
+    } else {
+        tcg_gen_ext32u_i64(v, cpu_X[reg]);
+    }
+    return v;
+}
+
 /* Set ZF and NF based on a 64 bit result. This is alas fiddlier
  * than the 32 bit equivalent.
  */
@@ -278,6 +298,124 @@  static inline void gen_logic_CC(int sf, TCGv_i64 result)
 }
 
 /*
+ * Load/Store generators
+ */
+
+/*
+ *  Store from GPR register to memory
+ */
+static void do_gpr_st(DisasContext *s, TCGv_i64 source,
+                      TCGv_i64 tcg_addr, int size)
+{
+    g_assert(size <= 3);
+    tcg_gen_qemu_st_i64(source, tcg_addr, get_mem_index(s), MO_TE + size);
+}
+
+/*
+ * Load from memory to GPR register
+ */
+static void do_gpr_ld(DisasContext *s, TCGv_i64 dest, TCGv_i64 tcg_addr,
+                      int size, bool is_signed, bool extend)
+{
+    TCGMemOp memop =  MO_TE + size;
+
+    g_assert(size <= 3);
+
+    if (is_signed) {
+        memop += MO_SIGN;
+    }
+
+    tcg_gen_qemu_ld_i64(dest, tcg_addr, get_mem_index(s), memop);
+
+    if (extend && is_signed) {
+        g_assert(size < 3);
+        tcg_gen_ext32u_i64(dest, dest);
+    }
+}
+
+/*
+ * Store from FP register to memory
+ */
+static void do_fp_st(DisasContext *s, int srcidx, TCGv_i64 tcg_addr, int size)
+{
+    /* This writes the bottom N bits of a 128 bit wide vector to memory */
+    int freg_offs = offsetof(CPUARMState, vfp.regs[srcidx * 2]);
+    TCGv_i64 tmp = tcg_temp_new_i64();
+
+    if (size < 4) {
+        switch (size) {
+        case 0:
+            tcg_gen_ld8u_i64(tmp, cpu_env, freg_offs);
+            break;
+        case 1:
+            tcg_gen_ld16u_i64(tmp, cpu_env, freg_offs);
+            break;
+        case 2:
+            tcg_gen_ld32u_i64(tmp, cpu_env, freg_offs);
+            break;
+        case 3:
+            tcg_gen_ld_i64(tmp, cpu_env, freg_offs);
+            break;
+        }
+        tcg_gen_qemu_st_i64(tmp, tcg_addr, get_mem_index(s), MO_TE + size);
+    } else {
+        TCGv_i64 tcg_hiaddr = tcg_temp_new_i64();
+        tcg_gen_ld_i64(tmp, cpu_env, freg_offs);
+        tcg_gen_qemu_st_i64(tmp, tcg_addr, get_mem_index(s), MO_TEQ);
+        tcg_gen_qemu_st64(tmp, tcg_addr, get_mem_index(s));
+        tcg_gen_ld_i64(tmp, cpu_env, freg_offs + sizeof(float64));
+        tcg_gen_addi_i64(tcg_hiaddr, tcg_addr, 8);
+        tcg_gen_qemu_st_i64(tmp, tcg_hiaddr, get_mem_index(s), MO_TEQ);
+        tcg_temp_free_i64(tcg_hiaddr);
+    }
+
+    tcg_temp_free_i64(tmp);
+}
+
+/* Load from memory to FP register */
+static void do_fp_ld(DisasContext *s, int destidx, TCGv_i64 tcg_addr, int size)
+{
+    /* This always zero-extends and writes to a full 128 bit wide vector */
+    int freg_offs = offsetof(CPUARMState, vfp.regs[destidx * 2]);
+    TCGv_i64 tmplo = tcg_temp_new_i64();
+    TCGv_i64 tmphi;
+
+    if (size < 4) {
+        TCGMemOp memop =  MO_TE + size;
+        tmphi = tcg_const_i64(0);
+        tcg_gen_qemu_ld_i64(tmplo, tcg_addr, get_mem_index(s), memop);
+    } else {
+        TCGv_i64 tcg_hiaddr;
+        tmphi = tcg_temp_new_i64();
+        tcg_hiaddr = tcg_temp_new_i64();
+
+        tcg_gen_qemu_ld_i64(tmplo, tcg_addr, get_mem_index(s), MO_TEQ);
+        tcg_gen_addi_i64(tcg_hiaddr, tcg_addr, 8);
+        tcg_gen_qemu_ld_i64(tmphi, tcg_hiaddr, get_mem_index(s), MO_TEQ);
+        tcg_temp_free_i64(tcg_hiaddr);
+    }
+
+    tcg_gen_st_i64(tmplo, cpu_env, freg_offs);
+    tcg_gen_st_i64(tmphi, cpu_env, freg_offs + sizeof(float64));
+
+    tcg_temp_free_i64(tmplo);
+    tcg_temp_free_i64(tmphi);
+}
+
+static inline void gen_check_sp_alignment(DisasContext *s)
+{
+    /* The AArch64 architecture mandates that (if enabled via PSTATE
+     * or SCTLR bits) there is a check that SP is 16-aligned on every
+     * SP-relative load or store (with an exception generated if it is not).
+     * In line with general QEMU practice regarding misaligned accesses,
+     * we omit these checks for the sake of guest program performance.
+     * This function is provided as a hook so we can more easily add these
+     * checks in future (possibly as a "favour catching guest program bugs
+     * over speed" user selectable option).
+     */
+}
+
+/*
  * the instruction disassembly implemented here matches
  * the instruction encoding classifications in chapter 3 (C3)
  * of the ARM Architecture Reference Manual (DDI0487A_a)
@@ -620,10 +758,130 @@  static void disas_ld_lit(DisasContext *s, uint32_t insn)
     unsupported_encoding(s, insn);
 }
 
-/* Load/store pair (all forms) */
-static void disas_ldst_pair(DisasContext *s, uint32_t insn)
-{
-    unsupported_encoding(s, insn);
+/*
+ * C5.6.81 LDP (Load Pair - non vector)
+ * C5.6.82 LDPSW (Load Pair Signed Word - non vector)
+ * C5.6.177 STP (Store Pair - non vector)
+ * C6.3.165 LDP (Load Pair of SIMD&FP)
+ * C6.3.284 STP (Store Pair of SIMD&FP)
+ *
+ *  31 30 29   27  26 25   23  22 21   15 14   10 9    5 4    0
+ * +-----+-------+---+-------+---+-----------------------------+
+ * | opc | 1 0 1 | V | index | L |  imm7 |  Rt2  |  Rn  | Rt   |
+ * +-----+-------+---+-------+---+-------+-------+------+------+
+ *
+ * opc: LDP&STP       00 -> 32 bit, 10 -> 64 bit
+ *      LDPSW         01
+ *      LDP&STP(SIMD) 00 -> 32 bit, 01 -> 64 bit, 10 -> 128 bit
+ *   V: 0 -> GPR, 1 -> Vector
+ * idx: 001 -> post-index, 011 -> pre-index, 010 -> signed off
+ *   L: 0 -> Store 1 -> Load
+ *
+ * Rt, Rt2 = GPR or SIMD registers to be stored
+ * Rn = general purpose register containing address
+ * imm7 = signed offset (multiple of 4 or 8 depending on size)
+ */
+static void handle_ldst_pair(DisasContext *s, uint32_t insn)
+{
+    int rt = extract32(insn, 0, 5);
+    int rn = extract32(insn, 5, 5);
+    int rt2 = extract32(insn, 10, 5);
+    int64_t offset = sextract32(insn, 15, 7);
+    int index = extract32(insn, 23, 2);
+    bool is_vector = extract32(insn, 26, 1);
+    bool is_load = extract32(insn, 22, 1);
+    int opc = extract32(insn, 30, 2);
+
+    bool is_signed = false;
+    bool postindex = false;
+    bool wback = false;
+
+    TCGv_i64 tcg_addr; /* calculated address */
+    int size;
+
+    if (is_vector) {
+        if (opc == 3) {
+            unallocated_encoding(s);
+            return;
+        }
+        size = 2 + opc;
+    } else {
+        size = 2 + extract32(opc, 1, 1);
+        if (is_load) {
+            is_signed = opc & 1;
+        } else if (opc & 1) {
+            unallocated_encoding(s);
+            return;
+        }
+    }
+
+    switch (index) {
+    case 1: /* post-index */
+        postindex = true;
+        wback = true;
+        break;
+    case 2: /* signed offset, rn not updated */
+        postindex = false;
+        break;
+    case 3: /* pre-index */
+        postindex = false;
+        wback = true;
+        break;
+    default: /* Failed decoder tree? */
+        unallocated_encoding(s);
+        break;
+    }
+
+    offset <<= size;
+
+    if (rn == 31) {
+        gen_check_sp_alignment(s);
+    }
+
+    tcg_addr = read_cpu_reg_sp(s, rn, 1);
+
+    if (!postindex) {
+        tcg_gen_addi_i64(tcg_addr, tcg_addr, offset);
+    }
+
+    if (is_vector) {
+        if (is_load) {
+            do_fp_ld(s, rt, tcg_addr, size);
+        } else {
+            do_fp_st(s, rt, tcg_addr, size);
+        }
+    } else {
+        TCGv_i64 tcg_rt = cpu_reg(s, rt);
+        if (is_load) {
+            do_gpr_ld(s, tcg_rt, tcg_addr, size, is_signed, false);
+        } else {
+            do_gpr_st(s, tcg_rt, tcg_addr, size);
+        }
+    }
+    tcg_gen_addi_i64(tcg_addr, tcg_addr, 1 << size);
+    if (is_vector) {
+        if (is_load) {
+            do_fp_ld(s, rt2, tcg_addr, size);
+        } else {
+            do_fp_st(s, rt2, tcg_addr, size);
+        }
+    } else {
+        TCGv_i64 tcg_rt2 = cpu_reg(s, rt2);
+        if (is_load) {
+            do_gpr_ld(s, tcg_rt2, tcg_addr, size, is_signed, false);
+        } else {
+            do_gpr_st(s, tcg_rt2, tcg_addr, size);
+        }
+    }
+
+    if (wback) {
+        if (postindex) {
+            tcg_gen_addi_i64(tcg_addr, tcg_addr, offset - (1 << size));
+        } else {
+            tcg_gen_subi_i64(tcg_addr, tcg_addr, 1 << size);
+        }
+        tcg_gen_mov_i64(cpu_reg_sp(s, rn), tcg_addr);
+    }
 }
 
 /* Load/store register (all forms) */
@@ -656,7 +914,7 @@  static void disas_ldst(DisasContext *s, uint32_t insn)
         break;
     case 0x28: case 0x29:
     case 0x2c: case 0x2d: /* Load/store pair (all forms) */
-        disas_ldst_pair(s, insn);
+        handle_ldst_pair(s, insn);
         break;
     case 0x38: case 0x39:
     case 0x3c: case 0x3d: /* Load/store register (all forms) */