Re: [PATCH] KVM: nSVM: fix nested tsc scaling when not enabled but MSR_AMD64_TSC_RATIO set

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



On 2/21/22 11:33, Maxim Levitsky wrote:
For compatibility with userspace the MSR_AMD64_TSC_RATIO can be set
even when the feature is not exposed to the guest, which breaks assumptions
that it has the default value in this case.

Fixes: 5228eb96a487 ("KVM: x86: nSVM: implement nested TSC scaling")
Cc: stable@xxxxxxxxxxxxxxx

Reported-by: Dāvis Mosāns <davispuh@xxxxxxxxx>
Signed-off-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>

It's not clear how QEMU ends up writing MSR_AMD64_TSC_RATIO_DEFAULT rather than 0, but we clearly have a bug in KVM. It should not allow writing 0 in the first place if tsc-ratio is not available in the VM.

If QEMU really can get itself in this situation, we cannot fix this except with KVM_CAP_DISABLE_QUIRKS (a quirk that would accept and ignore host-initiated writes if the CPUID bit is not available) or perhaps with a pr_warn_ratelimited and a quick deprecation cycle, until some time after 6.2.1 is released.

Hmmm... maybe the issue is actually that KVM *returns* 0 from KVM_GET_MSRS? And in this case, fixing KVM would also prevent QEMU from sending the bogus KVM_SET_MSRS?

Thanks,

Paolo

---
  arch/x86/kvm/svm/nested.c | 10 ++++------
  arch/x86/kvm/svm/svm.c    |  5 +++--
  arch/x86/kvm/svm/svm.h    |  1 +
  3 files changed, 8 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index 1218b5a342fc..a2e2436057dc 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -574,14 +574,12 @@ static void nested_vmcb02_prepare_control(struct vcpu_svm *svm)
  	vcpu->arch.tsc_offset = kvm_calc_nested_tsc_offset(
  			vcpu->arch.l1_tsc_offset,
  			svm->nested.ctl.tsc_offset,
-			svm->tsc_ratio_msr);
+			svm_get_l2_tsc_multiplier(vcpu));
svm->vmcb->control.tsc_offset = vcpu->arch.tsc_offset; - if (svm->tsc_ratio_msr != kvm_default_tsc_scaling_ratio) {
-		WARN_ON(!svm->tsc_scaling_enabled);
+	if (svm_get_l2_tsc_multiplier(vcpu) != kvm_default_tsc_scaling_ratio)
  		nested_svm_update_tsc_ratio_msr(vcpu);
-	}
svm->vmcb->control.int_ctl =
  		(svm->nested.ctl.int_ctl & int_ctl_vmcb12_bits) |
@@ -867,8 +865,7 @@ int nested_svm_vmexit(struct vcpu_svm *svm)
  		vmcb_mark_dirty(svm->vmcb, VMCB_INTERCEPTS);
  	}
- if (svm->tsc_ratio_msr != kvm_default_tsc_scaling_ratio) {
-		WARN_ON(!svm->tsc_scaling_enabled);
+	if (svm_get_l2_tsc_multiplier(vcpu) != kvm_default_tsc_scaling_ratio) {
  		vcpu->arch.tsc_scaling_ratio = vcpu->arch.l1_tsc_scaling_ratio;
  		svm_write_tsc_multiplier(vcpu, vcpu->arch.tsc_scaling_ratio);
  	}
@@ -1264,6 +1261,7 @@ void nested_svm_update_tsc_ratio_msr(struct kvm_vcpu *vcpu)
  {
  	struct vcpu_svm *svm = to_svm(vcpu);
+ WARN_ON_ONCE(!svm->tsc_scaling_enabled);
  	vcpu->arch.tsc_scaling_ratio =
  		kvm_calc_nested_tsc_multiplier(vcpu->arch.l1_tsc_scaling_ratio,
  					       svm->tsc_ratio_msr);
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 975be872cd1a..5cf6bb5bfd3e 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -911,11 +911,12 @@ static u64 svm_get_l2_tsc_offset(struct kvm_vcpu *vcpu)
  	return svm->nested.ctl.tsc_offset;
  }
-static u64 svm_get_l2_tsc_multiplier(struct kvm_vcpu *vcpu)
+u64 svm_get_l2_tsc_multiplier(struct kvm_vcpu *vcpu)
  {
  	struct vcpu_svm *svm = to_svm(vcpu);
- return svm->tsc_ratio_msr;
+	return svm->tsc_scaling_enabled ? svm->tsc_ratio_msr :
+	       kvm_default_tsc_scaling_ratio;
  }
static void svm_write_tsc_offset(struct kvm_vcpu *vcpu, u64 offset)
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index 852b12aee03d..006571dd30df 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -542,6 +542,7 @@ int nested_svm_check_exception(struct vcpu_svm *svm, unsigned nr,
  			       bool has_error_code, u32 error_code);
  int nested_svm_exit_special(struct vcpu_svm *svm);
  void nested_svm_update_tsc_ratio_msr(struct kvm_vcpu *vcpu);
+u64 svm_get_l2_tsc_multiplier(struct kvm_vcpu *vcpu);
  void svm_write_tsc_multiplier(struct kvm_vcpu *vcpu, u64 multiplier);
  void nested_copy_vmcb_control_to_cache(struct vcpu_svm *svm,
  				       struct vmcb_control_area *control);




[Index of Archives]     [KVM ARM]     [KVM ia64]     [KVM ppc]     [Virtualization Tools]     [Spice Development]     [Libvirt]     [Libvirt Users]     [Linux USB Devel]     [Linux Audio Users]     [Yosemite Questions]     [Linux Kernel]     [Linux SCSI]     [XFree86]

  Powered by Linux