diff mbox

[TSC,emulation,5/9] Add last_host_tsc tracking back to KVM

Message ID 4E008CF6.1000601@redhat.com (mailing list archive)
State New, archived
Headers show

Commit Message

Zachary Amsden June 21, 2011, 12:22 p.m. UTC 
-------- Original Message --------
Subject: 	[KVM TSC emulation 5/9] Add last_host_tsc tracking back to KVM
Date: 	Mon, 20 Jun 2011 16:59:33 -0700
From: 	Zachary Amsden <zamsden@redhat.com>
To: 	Avi Kivity <avi@redhat.com>, Marcelo Tosatti <mtosatti@redhat.com>, 
Glauber Costa <glommer@redhat.com>, Frank Arnold <farnold@redhat.com>, 
Joerg Roedel <joerg.roedel@amd.com>, Jan Kiszka 
<jan.kiszka@siemens.com>, linux-kvm@vger.kernel.org, 
linux-kernel@vger.kernel.org, Zachary Amsden <zamsden@gmail.com>, Avi 
Kivity <avi@redhat.com>, Marcelo Tosatti <mtosatti@redhat.com>, Glauber 
Costa <glommer@redhat.com>, Frank Arnold <farnold@redhat.com>, Joerg 
Roedel <joerg.roedel@amd.com>, Jan Kiszka <jan.kiszka@siemens.com>, 
linux-kvm@vger.kernel.org
CC: 	Zachary Amsden <zamsden@redhat.com>, Zachary Amsden 
<zamsden@gmail.com>



The variable last_host_tsc was removed from upstream code.  I am adding
it back for two reasons.  First, it is unnecessary to use guest TSC
computation to conclude information about the host TSC.  The guest may
set the TSC backwards (this case handled by the previous patch), but
the computation of guest TSC (and fetching an MSR) is significanlty more
work and complexity than simply reading the hardware counter.  In addition,
we don't actually need the guest TSC for any part of the computation,
by always recomputing the offset, we can eliminate the need to deal with
the current offset and any scaling factors that may apply.

The second reason is that later on, we are going to be using the host
TSC value to restore TSC offsets after a host S4 suspend, so we need to
be reading the host values, not the guest values here.

Signed-off-by: Zachary Amsden<zamsden@redhat.com>
---
  arch/x86/include/asm/kvm_host.h |    1 +
  arch/x86/kvm/x86.c              |    9 +++------
  2 files changed, 4 insertions(+), 6 deletions(-)
diff mbox

Patch

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 865f051..3b9fdb5 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -393,6 +393,7 @@  struct kvm_vcpu_arch {
  	u64 last_kernel_ns;
  	u64 last_tsc_nsec;
  	u64 last_tsc_write;
+	u64 last_host_tsc;
  	bool tsc_catchup;
  	bool tsc_always_catchup;
  	s8 virtual_tsc_shift;
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 3bd2ca3..8fe988a 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2154,11 +2154,8 @@  void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
  	kvm_x86_ops->vcpu_load(vcpu, cpu);
  	if (unlikely(vcpu->cpu != cpu) || check_tsc_unstable()) {
  		/* Make sure TSC doesn't go backwards */
-		s64 tsc_delta;
-		u64 tsc;
-
-		kvm_get_msr(vcpu, MSR_IA32_TSC,&tsc);
-		tsc_delta = tsc - vcpu->arch.last_guest_tsc;
+		s64 tsc_delta = !vcpu->arch.last_host_tsc ? 0 :
+				native_read_tsc() - vcpu->arch.last_host_tsc;
  		if (tsc_delta<  0)
  			mark_tsc_unstable("KVM discovered backwards TSC");
  		if (check_tsc_unstable()) {
@@ -2178,7 +2175,7 @@  void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
  {
  	kvm_x86_ops->vcpu_put(vcpu);
  	kvm_put_guest_fpu(vcpu);
-	kvm_get_msr(vcpu, MSR_IA32_TSC,&vcpu->arch.last_guest_tsc);
+	vcpu->arch.last_host_tsc = native_read_tsc();
  }

  static int is_efer_nx(void)