[patch 3/3] KVM: VMX: initialize TSC offset relative to vm creation time

Marcelo Tosatti
2008-12-09T01:26:02Z

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VMX initializes the TSC offset for each vcpu at different times, and
also reinitializes it for vcpus other than 0 on APIC SIPI message.

This bug causes the TSC's to appear unsynchronized in the guest, even if
the host is good.

Older Linux kernels don't handle the situation very well, so
gettimeofday is likely to go backwards in time:

http://www.mail-archive.com/kvm@vger.kernel.org/msg02955.html
http://sourceforge.net/tracker/index.php?func=detail&aid=2025534&group_id=180599&atid=893831

Fix it by initializating the offset of each vcpu relative to vm creation
time, and moving it from vmx_vcpu_reset to vmx_vcpu_setup, out of the
APIC MP init path.

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

Index: kvm/arch/x86/include/asm/kvm_host.h
===================================================================
--- kvm.orig/arch/x86/include/asm/kvm_host.h
+++ kvm/arch/x86/include/asm/kvm_host.h
@@ -378,6 +378,7 @@ struct kvm_arch{

unsigned long irq_sources_bitmap;
unsigned long irq_states[KVM_IOAPIC_NUM_PINS];
+ u64 vm_init_tsc;
};

struct kvm_vm_stat {
Index: kvm/arch/x86/kvm/vmx.c
===================================================================
--- kvm.orig/arch/x86/kvm/vmx.c
+++ kvm/arch/x86/kvm/vmx.c
@@ -856,11 +856,8 @@ static u64 guest_read_tsc(void)
* writes 'guest_tsc' into guest's timestamp counter "register"
* guest_tsc = host_tsc + tsc_offset ==> tsc_offset = guest_tsc - host_tsc
*/
-static void guest_write_tsc(u64 guest_tsc)
+static void guest_write_tsc(u64 guest_tsc, u64 host_tsc)
{
- u64 host_tsc;
-
- rdtscll(host_tsc);
vmcs_write64(TSC_OFFSET, guest_tsc - host_tsc);
}

@@ -924,6 +921,7 @@ static int vmx_set_msr(struct kvm_vcpu *
{
struct vcpu_vmx *vmx = to_vmx(vcpu);
struct kvm_msr_entry *msr;
+ u64 host_tsc;
int ret = 0;

switch (msr_index) {
@@ -949,7 +947,8 @@ static int vmx_set_msr(struct kvm_vcpu *
vmcs_writel(GUEST_SYSENTER_ESP, data);
break;
case MSR_IA32_TIME_STAMP_COUNTER:
- guest_write_tsc(data);
+ rdtscll(host_tsc);
+ guest_write_tsc(data, host_tsc);
break;
case MSR_P6_PERFCTR0:
case MSR_P6_PERFCTR1:
@@ -2239,6 +2238,7 @@ static int vmx_vcpu_setup(struct vcpu_vm
vmcs_writel(CR0_GUEST_HOST_MASK, ~0UL);
vmcs_writel(CR4_GUEST_HOST_MASK, KVM_GUEST_CR4_MASK);

+ guest_write_tsc(0, vmx->vcpu.kvm->arch.vm_init_tsc);

return 0;
}
@@ -2331,8 +2331,6 @@ static int vmx_vcpu_reset(struct kvm_vcp
vmcs_write32(GUEST_INTERRUPTIBILITY_INFO, 0);
vmcs_write32(GUEST_PENDING_DBG_EXCEPTIONS, 0);

- guest_write_tsc(0);
-
/* Special registers */
vmcs_write64(GUEST_IA32_DEBUGCTL, 0);

Index: kvm/arch/x86/kvm/x86.c
===================================================================
--- kvm.orig/arch/x86/kvm/x86.c
+++ kvm/arch/x86/kvm/x86.c
@@ -4122,6 +4122,8 @@ struct kvm *kvm_arch_create_vm(void)
/* Reserve bit 0 of irq_sources_bitmap for userspace irq source */
set_bit(KVM_USERSPACE_IRQ_SOURCE_ID, &kvm->arch.irq_sources_bitmap);

+ rdtscll(kvm->arch.vm_init_tsc);
+
return kvm;
}

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