| Message ID | 20211123173759.1383510-3-richard.henderson@linaro.org |
|---|---|
| State | New |
| Headers | show |
| Series | None | expand |
On Tue, 23 Nov 2021 at 17:40, Richard Henderson <richard.henderson@linaro.org> wrote: > > We don't need a register that can live across the syscall; > we only need a register that can live until the syscall. What about the case where: * we execute the sc instruction (r11 trashed) * the syscall is one that from the host kernel point of view is restartable * the kernel arranges to restart the syscall by rewinding the PC to point to the start of the 'sc' instruction * our rewind_if_in_safe_syscall() rewinds PC further to point at safe_syscall_start * we want to use r11 again, but it was trashed in step 1 ? Put another way, this patch is effectively a revert of commit 5d9f3ea081721, which was a fix to an observed bug. -- PMM
On 11/29/21 12:01 PM, Peter Maydell wrote: > On Tue, 23 Nov 2021 at 17:40, Richard Henderson > <richard.henderson@linaro.org> wrote: >> >> We don't need a register that can live across the syscall; >> we only need a register that can live until the syscall. > > What about the case where: > * we execute the sc instruction (r11 trashed) > * the syscall is one that from the host kernel point of > view is restartable > * the kernel arranges to restart the syscall by rewinding the > PC to point to the start of the 'sc' instruction > * our rewind_if_in_safe_syscall() rewinds PC further to > point at safe_syscall_start > * we want to use r11 again, but it was trashed in step 1 > ? > > Put another way, this patch is effectively a revert of > commit 5d9f3ea081721, which was a fix to an observed bug. Whoops. I forgot about that (a mere 3 years ago). r~
diff --git a/linux-user/host/ppc64/safe-syscall.inc.S b/linux-user/host/ppc64/safe-syscall.inc.S index 4b57440585..5f19cd193c 100644 --- a/linux-user/host/ppc64/safe-syscall.inc.S +++ b/linux-user/host/ppc64/safe-syscall.inc.S @@ -49,9 +49,7 @@ safe_syscall_base: * and returns the result in r3 * Shuffle everything around appropriately. */ - std 14, 16(1) /* Preserve r14 in SP+16 */ - .cfi_offset 14, 16 - mr 14, 3 /* signal_pending */ + mr 11, 3 /* signal_pending */ mr 0, 4 /* syscall number */ mr 3, 5 /* syscall arguments */ mr 4, 6 @@ -69,13 +67,12 @@ safe_syscall_base: */ safe_syscall_start: /* if signal_pending is non-zero, don't do the call */ - lwz 12, 0(14) + lwz 12, 0(11) cmpwi 0, 12, 0 bne- 0f sc safe_syscall_end: /* code path when we did execute the syscall */ - ld 14, 16(1) /* restore r14 to its original value */ bnslr+ /* syscall failed; return negative errno */ @@ -84,7 +81,6 @@ safe_syscall_end: /* code path when we didn't execute the syscall */ 0: addi 3, 0, -TARGET_ERESTARTSYS - ld 14, 16(1) /* restore r14 to its original value */ blr .cfi_endproc
We don't need a register that can live across the syscall; we only need a register that can live until the syscall. Use call-clobbered r11 instead of call-saved r14. Eliminate the save and restore of r14 from the stack frame. Signed-off-by: Richard Henderson <richard.henderson@linaro.org> --- linux-user/host/ppc64/safe-syscall.inc.S | 8 ++------ 1 file changed, 2 insertions(+), 6 deletions(-)