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Submitter Stefan Bader
Date March 11, 2011, 2:41 p.m.
Message ID <1299854503-27699-1-git-send-email-stefan.bader@canonical.com>
Download mbox | patch
Permalink /patch/86425/
State New
Headers show

Pull-request

git://kernel.ubuntu.com/smb/linux-2.6.32.y.git kvm-next

Comments

Stefan Bader - March 11, 2011, 2:41 p.m.
Serge,

I would ask the KVM maintainers about pushing your backports to stable.
So I prepared a branch to pull from with your patches (slightly modified
in the message parts). This is when I noticed the last round you sent
had #/4 in the subject. Though the original post said three patches, I
just want to make sure I did not miss anything and you are ok with how
they look now and me forwarding them to the maintainers in your name.

-Stefan

The following changes since commit 0533a1fed29c6b890599647d747439f2c2af1415:
  Greg Kroah-Hartman (1):
        Linux 2.6.32.32

are available in the git repository at:

  git://kernel.ubuntu.com/smb/linux-2.6.32.y.git kvm-next

Serge E. Hallyn (2):
      KVM: x86: Fix a possible backwards warp of kvmclock
      KVM: x86: Fix kvmclock bug

Zachary Amsden (1):
      x86: pvclock: Move scale_delta into common header

 arch/x86/include/asm/kvm_host.h |    3 ++
 arch/x86/include/asm/pvclock.h  |   38 ++++++++++++++++++++++++++++++
 arch/x86/kernel/pvclock.c       |    3 +-
 arch/x86/kvm/x86.c              |   48 ++++++++++++++++++++++++++++++++++++--
 4 files changed, 88 insertions(+), 4 deletions(-)

---

From 7e6d51377bda5146118fb58b85556aa3ce83c0b6 Mon Sep 17 00:00:00 2001
From: Zachary Amsden <zamsden@redhat.com>
Date: Thu, 19 Aug 2010 22:07:29 -1000
Subject: [PATCH 1/3] x86: pvclock: Move scale_delta into common header

The scale_delta function for shift / multiply with 31-bit
precision moves to a common header so it can be used by both
kernel and kvm module.

Signed-off-by: Zachary Amsden <zamsden@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

BugLink: http://bugs.launchpad.net/bugs/714335

(cherry-picked from commit 347bb4448c2155eb2310923ccaa4be5677649003)
Signed-off-by: Serge E. Hallyn <serge.hallyn@canonical.com>
---
 arch/x86/include/asm/pvclock.h |   38 ++++++++++++++++++++++++++++++++++++++
 arch/x86/kernel/pvclock.c      |    3 ++-
 2 files changed, 40 insertions(+), 1 deletions(-)

Patch

diff --git a/arch/x86/include/asm/pvclock.h b/arch/x86/include/asm/pvclock.h
index daaacab..982aa32 100644
--- a/arch/x86/include/asm/pvclock.h
+++ b/arch/x86/include/asm/pvclock.h
@@ -12,4 +12,42 @@  void pvclock_read_wallclock(struct pvclock_wall_clock *wall,
 			    struct timespec *ts);
 void pvclock_resume(void);
 
+/*
+ * Scale a 64-bit delta by scaling and multiplying by a 32-bit fraction,
+ * yielding a 64-bit result.
+ */
+static inline u64 pvclock_scale_delta(u64 delta, u32 mul_frac, int shift)
+{
+	u64 product;
+#ifdef __i386__
+	u32 tmp1, tmp2;
+#endif
+
+	if (shift < 0)
+		delta >>= -shift;
+	else
+		delta <<= shift;
+
+#ifdef __i386__
+	__asm__ (
+		"mul  %5       ; "
+		"mov  %4,%%eax ; "
+		"mov  %%edx,%4 ; "
+		"mul  %5       ; "
+		"xor  %5,%5    ; "
+		"add  %4,%%eax ; "
+		"adc  %5,%%edx ; "
+		: "=A" (product), "=r" (tmp1), "=r" (tmp2)
+		: "a" ((u32)delta), "1" ((u32)(delta >> 32)), "2" (mul_frac) );
+#elif defined(__x86_64__)
+	__asm__ (
+		"mul %%rdx ; shrd $32,%%rdx,%%rax"
+		: "=a" (product) : "0" (delta), "d" ((u64)mul_frac) );
+#else
+#error implement me!
+#endif
+
+	return product;
+}
+
 #endif /* _ASM_X86_PVCLOCK_H */
diff --git a/arch/x86/kernel/pvclock.c b/arch/x86/kernel/pvclock.c
index b12fe8d..929047c 100644
--- a/arch/x86/kernel/pvclock.c
+++ b/arch/x86/kernel/pvclock.c
@@ -74,7 +74,8 @@  static inline u64 scale_delta(u64 delta, u32 mul_frac, int shift)
 static u64 pvclock_get_nsec_offset(struct pvclock_shadow_time *shadow)
 {
 	u64 delta = native_read_tsc() - shadow->tsc_timestamp;
-	return scale_delta(delta, shadow->tsc_to_nsec_mul, shadow->tsc_shift);
+	return pvclock_scale_delta(delta, shadow->tsc_to_nsec_mul,
+				   shadow->tsc_shift);
 }
 
 /*
-- 
1.7.0.4

From 06bd89325e6441c130c8dc71859867f6d68d617c Mon Sep 17 00:00:00 2001
From: Serge E. Hallyn <serge.hallyn@canonical.com>
Date: Wed, 2 Mar 2011 20:05:34 +0000
Subject: [PATCH 2/3] KVM: x86: Fix a possible backwards warp of kvmclock

Kernel time, which advances in discrete steps may progress much slower
than TSC.  As a result, when kvmclock is adjusted to a new base, the
apparent time to the guest, which runs at a much higher, nsec scaled
rate based on the current TSC, may have already been observed to have
a larger value (kernel_ns + scaled tsc) than the value to which we are
setting it (kernel_ns + 0).

We must instead compute the clock as potentially observed by the guest
for kernel_ns to make sure it does not go backwards.

Signed-off-by: Zachary Amsden <zamsden@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

BugLink: http://bugs.launchpad.net/bugs/714335

(backported from commit 1d5f066e0b63271b67eac6d3752f8aa96adcbddb)
Signed-off-by: Serge E. Hallyn <serge.hallyn@canonical.com>
Reviewed-by: Stefan Bader <stefan.bader@canonical.com>
---
 arch/x86/include/asm/kvm_host.h |    3 ++
 arch/x86/kvm/x86.c              |   47 ++++++++++++++++++++++++++++++++++++--
 2 files changed, 47 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 600807b..08bc2ff 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -357,6 +357,9 @@  struct kvm_vcpu_arch {
 	struct page *time_page;
 
 	bool singlestep; /* guest is single stepped by KVM */
+	u64 last_guest_tsc;
+	u64 last_kernel_ns;
+
 	bool nmi_pending;
 	bool nmi_injected;
 
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index b2c02a2..8a10f27 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -47,6 +47,7 @@ 
 #include <asm/desc.h>
 #include <asm/mtrr.h>
 #include <asm/mce.h>
+#include <asm/pvclock.h>
 
 #define MAX_IO_MSRS 256
 #define CR0_RESERVED_BITS						\
@@ -633,6 +634,8 @@  static void kvm_write_guest_time(struct kvm_vcpu *v)
 	struct kvm_vcpu_arch *vcpu = &v->arch;
 	void *shared_kaddr;
 	unsigned long this_tsc_khz;
+	s64 kernel_ns, max_kernel_ns;
+	u64 tsc_timestamp;
 
 	if ((!vcpu->time_page))
 		return;
@@ -646,15 +649,51 @@  static void kvm_write_guest_time(struct kvm_vcpu *v)
 
 	/* Keep irq disabled to prevent changes to the clock */
 	local_irq_save(flags);
-	kvm_get_msr(v, MSR_IA32_TSC, &vcpu->hv_clock.tsc_timestamp);
+	kvm_get_msr(v, MSR_IA32_TSC, &tsc_timestamp);
 	ktime_get_ts(&ts);
 	monotonic_to_bootbased(&ts);
+	kernel_ns = timespec_to_ns(&ts);
 	local_irq_restore(flags);
 
+	/*
+	 * Time as measured by the TSC may go backwards when resetting the base
+	 * tsc_timestamp.  The reason for this is that the TSC resolution is
+	 * higher than the resolution of the other clock scales.  Thus, many
+	 * possible measurments of the TSC correspond to one measurement of any
+	 * other clock, and so a spread of values is possible.  This is not a
+	 * problem for the computation of the nanosecond clock; with TSC rates
+	 * around 1GHZ, there can only be a few cycles which correspond to one
+	 * nanosecond value, and any path through this code will inevitably
+	 * take longer than that.  However, with the kernel_ns value itself,
+	 * the precision may be much lower, down to HZ granularity.  If the
+	 * first sampling of TSC against kernel_ns ends in the low part of the
+	 * range, and the second in the high end of the range, we can get:
+	 *
+	 * (TSC - offset_low) * S + kns_old > (TSC - offset_high) * S + kns_new
+	 *
+	 * As the sampling errors potentially range in the thousands of cycles,
+	 * it is possible such a time value has already been observed by the
+	 * guest.  To protect against this, we must compute the system time as
+	 * observed by the guest and ensure the new system time is greater.
+	 */
+	max_kernel_ns = 0;
+	if (vcpu->hv_clock.tsc_timestamp && vcpu->last_guest_tsc) {
+		max_kernel_ns = vcpu->last_guest_tsc -
+			vcpu->hv_clock.tsc_timestamp;
+		max_kernel_ns = pvclock_scale_delta(max_kernel_ns,
+				vcpu->hv_clock.tsc_to_system_mul,
+				vcpu->hv_clock.tsc_shift);
+		max_kernel_ns += vcpu->last_kernel_ns;
+	}
+
+	if (max_kernel_ns > kernel_ns)
+		kernel_ns = max_kernel_ns;
+
 	/* With all the info we got, fill in the values */
 
-	vcpu->hv_clock.system_time = ts.tv_nsec +
-				     (NSEC_PER_SEC * (u64)ts.tv_sec) + v->kvm->arch.kvmclock_offset;
+	vcpu->hv_clock.tsc_timestamp = tsc_timestamp;
+	vcpu->hv_clock.system_time = kernel_ns + v->kvm->arch.kvmclock_offset;
+	vcpu->last_kernel_ns = kernel_ns;
 
 	/*
 	 * The interface expects us to write an even number signaling that the
@@ -3695,6 +3734,8 @@  static int vcpu_enter_guest(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
 	kvm_x86_ops->prepare_guest_switch(vcpu);
 	kvm_load_guest_fpu(vcpu);
 
+	kvm_get_msr(vcpu, MSR_IA32_TSC, &vcpu->arch.last_guest_tsc);
+
 	local_irq_disable();
 
 	clear_bit(KVM_REQ_KICK, &vcpu->requests);
-- 
1.7.0.4

From 36cf90d9adc1fcdd98eeb89b97948882fbd2d1db Mon Sep 17 00:00:00 2001
From: Serge E. Hallyn <serge.hallyn@canonical.com>
Date: Wed, 2 Mar 2011 20:07:48 +0000
Subject: [PATCH 3/3] KVM: x86: Fix kvmclock bug

If preempted after kvmclock values are updated, but before hardware
virtualization is entered, the last tsc time as read by the guest is
never set.  It underflows the next time kvmclock is updated if there
has not yet been a successful entry / exit into hardware virt.

Fix this by simply setting last_tsc to the newly read tsc value so
that any computed nsec advance of kvmclock is nulled.

Signed-off-by: Zachary Amsden <zamsden@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

BugLink: http://bugs.launchpad.net/bugs/714335

(backported from commit 28e4639adf0c9f26f6bb56149b7ab547bf33bb95)
Signed-off-by: Serge E. Hallyn <serge.hallyn@canonical.com>
Reviewed-by: Stefan Bader <stefan.bader@canonical.com>
---
 arch/x86/kvm/x86.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 8a10f27..df1cefb 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -694,6 +694,7 @@  static void kvm_write_guest_time(struct kvm_vcpu *v)
 	vcpu->hv_clock.tsc_timestamp = tsc_timestamp;
 	vcpu->hv_clock.system_time = kernel_ns + v->kvm->arch.kvmclock_offset;
 	vcpu->last_kernel_ns = kernel_ns;
+	vcpu->last_guest_tsc = tsc_timestamp;
 
 	/*
 	 * The interface expects us to write an even number signaling that the