KVM: x86: Reset tsc_timestamp on TSC writes

There is no upstream commit ID for this patch since it is not a straight
backport from upstream. It is a fix only relevant to 2.6.32.y.

Since 1d5f066e0b63271b67eac6d3752f8aa96adcbddb from 2.6.37 was
back-ported to 2.6.32.40 as ad2088cabe0fd7f633f38ba106025d33ed9a2105,
the following patch is needed to add the needed reset logic to 2.6.32 as
well.

Bug #23257: Reset tsc_timestamp on TSC writes

vcpu->last_guest_tsc is updated in vcpu_enter_guest() and kvm_arch_vcpu_put()
by getting the last value of the TSC from the guest.
On reset, the SeaBIOS resets the TSC to 0, which triggers a bug on the next
call to kvm_write_guest_time(): Since vcpu->hw_clock.tsc_timestamp still
contains the old value before the reset, "max_kernel_ns = vcpu->last_guest_tsc
- vcpu->hw_clock.tsc_timestamp" gets negative. Since the variable is u64, it
 gets translated to a large positive value.

[9333.197080]
vcpu->last_guest_tsc        =209_328_760_015           ←
vcpu->hv_clock.tsc_timestamp=209_328_708_109
vcpu->last_kernel_ns        =9_333_179_830_643
kernel_ns                   =9_333_197_073_429
max_kernel_ns               =9_333_179_847_943         ←

[9336.910995]
vcpu->last_guest_tsc        =9_438_510_584             ←
vcpu->hv_clock.tsc_timestamp=211_080_593_143
vcpu->last_kernel_ns        =9_333_763_732_907
kernel_ns                   =9_336_910_990_771
max_kernel_ns               =6_148_296_831_006_663_830 ←

For completeness, here are the values for my 3 GHz CPU:
vcpu->hv_clock.tsc_shift         =-1
vcpu->hv_clock.tsc_to_system_mul =2_863_019_502

This makes the guest kernel crawl very slowly when clocksource=kvmclock is
used: sleeps take way longer than expected and don't match wall clock any more.
The times printed with printk() don't match real time and the reboot often
stalls for long times.

In linux-git this isn't a problem, since on every MSR_IA32_TSC write
vcpu->arch.hv_clock.tsc_timestamp is reset to 0, which disables above logic.
The code there is only in arch/x86/kvm/x86.c, since much of the kvm-clock
related code has been refactured for 2.6.37:
	99e3e30a arch/x86/kvm/x86.c
        (Zachary Amsden            2010-08-19 22:07:17 -1000 1084)
        vcpu->arch.hv_clock.tsc_timestamp = 0;

Signed-off-by: Philipp Hahn <hahn@univention.de>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 253153d2e3e2b18b0c448b71170afd9ea63220eb..7c6e63e56c30255f5a74b244aed4f69a28ffdee2 100644 (file)
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -2256,6 +2256,7 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, unsigned ecx, u64 data)
                }
 
                svm->vmcb->control.tsc_offset = tsc_offset + g_tsc_offset;
+               vcpu->arch.hv_clock.tsc_timestamp = 0;
 
                break;
        }

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index d9c4fb6f0ff37545f95c29c520c3c9dde17c06c5..e6d925f8579a83ffa7a547d628d7548e0a045888 100644 (file)
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -1067,6 +1067,7 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, u32 msr_index, u64 data)
        case MSR_IA32_TSC:
                rdtscll(host_tsc);
                guest_write_tsc(data, host_tsc);
+               vcpu->arch.hv_clock.tsc_timestamp = 0;
                break;
        case MSR_IA32_CR_PAT:
                if (vmcs_config.vmentry_ctrl & VM_ENTRY_LOAD_IA32_PAT) {