[PATCH] kvm: svm: move WARN_ON in svm_adjust_tsc_offset

[PATCH] kvm: svm: move WARN_ON in svm_adjust_tsc_offset

[Chris J Arges]

When running the tsc_adjust kvm-unit-test on an AMD processor with the
IA32_TSC_ADJUST feature enabled, the WARN_ON in svm_adjust_tsc_offset can be
triggered. This WARN_ON checks for a negative adjustment in case __scale_tsc
is called; however it may trigger unnecessary warnings.

This patch moves the WARN_ON to trigger only if __scale_tsc will actually be
called from svm_adjust_tsc_offset. In addition make adj in kvm_set_msr_common
s64 since this can have signed values.

Signed-off-by: Chris J Arges <chris.j.arges@canonical.com>
---
 arch/x86/kvm/svm.c | 8 +++++---
 arch/x86/kvm/x86.c | 2 +-
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index d4f3aaa..6b411ad 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -1056,9 +1056,11 @@ static void svm_adjust_tsc_offset(struct kvm_vcpu *vcpu, s64 adjustment, bool ho
 {
 	struct vcpu_svm *svm = to_svm(vcpu);
 
-	WARN_ON(adjustment < 0);
-	if (host)
-		adjustment = svm_scale_tsc(vcpu, adjustment);
+	if (host) {
+		if (svm->tsc_ratio != TSC_RATIO_DEFAULT)
+			WARN_ON(adjustment < 0);
+		adjustment = svm_scale_tsc(vcpu, (u64)adjustment);
+	}
 
 	svm->vmcb->control.tsc_offset += adjustment;
 	if (is_guest_mode(vcpu))
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index f85da5c..1cd1376 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2141,7 +2141,7 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 	case MSR_IA32_TSC_ADJUST:
 		if (guest_cpuid_has_tsc_adjust(vcpu)) {
 			if (!msr_info->host_initiated) {
-				u64 adj = data - vcpu->arch.ia32_tsc_adjust_msr;
+				s64 adj = data - vcpu->arch.ia32_tsc_adjust_msr;
 				kvm_x86_ops->adjust_tsc_offset(vcpu, adj, true);
 			}
 			vcpu->arch.ia32_tsc_adjust_msr = data;
-- 
1.9.1

Re: [PATCH] kvm: svm: move WARN_ON in svm_adjust_tsc_offset

[Paolo Bonzini]

On 13/11/2014 04:00, Chris J Arges wrote:
> When running the tsc_adjust kvm-unit-test on an AMD processor with the
> IA32_TSC_ADJUST feature enabled, the WARN_ON in svm_adjust_tsc_offset can be
> triggered. This WARN_ON checks for a negative adjustment in case __scale_tsc
> is called; however it may trigger unnecessary warnings.
> 
> This patch moves the WARN_ON to trigger only if __scale_tsc will actually be
> called from svm_adjust_tsc_offset. In addition make adj in kvm_set_msr_common
> s64 since this can have signed values.
> 
> Signed-off-by: Chris J Arges <chris.j.arges@canonical.com>
> ---
>  arch/x86/kvm/svm.c | 8 +++++---
>  arch/x86/kvm/x86.c | 2 +-
>  2 files changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index d4f3aaa..6b411ad 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -1056,9 +1056,11 @@ static void svm_adjust_tsc_offset(struct kvm_vcpu *vcpu, s64 adjustment, bool ho
>  {
>  	struct vcpu_svm *svm = to_svm(vcpu);
>  
> -	WARN_ON(adjustment < 0);
> -	if (host)
> -		adjustment = svm_scale_tsc(vcpu, adjustment);
> +	if (host) {
> +		if (svm->tsc_ratio != TSC_RATIO_DEFAULT)
> +			WARN_ON(adjustment < 0);
> +		adjustment = svm_scale_tsc(vcpu, (u64)adjustment);
> +	}
>  
>  	svm->vmcb->control.tsc_offset += adjustment;
>  	if (is_guest_mode(vcpu))
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index f85da5c..1cd1376 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2141,7 +2141,7 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  	case MSR_IA32_TSC_ADJUST:
>  		if (guest_cpuid_has_tsc_adjust(vcpu)) {
>  			if (!msr_info->host_initiated) {
> -				u64 adj = data - vcpu->arch.ia32_tsc_adjust_msr;
> +				s64 adj = data - vcpu->arch.ia32_tsc_adjust_msr;
>  				kvm_x86_ops->adjust_tsc_offset(vcpu, adj, true);
>  			}
>  			vcpu->arch.ia32_tsc_adjust_msr = data;
> 

Applied, thanks!

Paolo