[PATCH v2] kvm: vmx: Pass through IA32_TSC_AUX for read iff guest has RDTSCP

[PATCH v2] kvm: vmx: Pass through IA32_TSC_AUX for read iff guest has RDTSCP

[Jim Mattson]

If the guest supports RDTSCP, it already has read access to the
hardware IA32_TSC_AUX MSR via RDTSCP, so we can allow it read access
via the RDMSR instruction as well. If the guest doesn't support
RDTSCP, intercept all accesses to the IA32_TSC_AUX MSR, so that kvm
can synthesize a #GP.  (IA32_TSC_AUX exists iff RDTSCP is supported.)

Signed-off-by: Jim Mattson <jmattson@google.com>
Reviewed-by: Marc Orr <marcorr@google.com>
Reviewed-by: Peter Shier <pshier@google.com>
Reviewed-by: Krish Sadhukhan <krish.sadhukhan@oracle.com>
Reviewed-by: Sean Christopherson <sean.j.christopherson@intel.com>

---
v1 -> v2: Rebased across vmx directory creation.
          Modified commit message based on Sean's comments.

 arch/x86/kvm/vmx/vmx.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index d175429c91b0..04a728976d96 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -4070,6 +4070,10 @@ static void vmx_compute_secondary_exec_control(struct vcpu_vmx *vmx)
 
 	if (vmx_rdtscp_supported()) {
 		bool rdtscp_enabled = guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP);
+
+		vmx_set_intercept_for_msr(vmx->vmcs01.msr_bitmap, MSR_TSC_AUX,
+					  MSR_TYPE_R, !rdtscp_enabled);
+
 		if (!rdtscp_enabled)
 			exec_control &= ~SECONDARY_EXEC_RDTSCP;
 
-- 
2.24.0.393.g34dc348eaf-goog

Re: [PATCH v2] kvm: vmx: Pass through IA32_TSC_AUX for read iff guest has RDTSCP

[Liran Alon]

> On 3 Dec 2019, at 20:35, Jim Mattson <jmattson@google.com> wrote:
> 
> If the guest supports RDTSCP, it already has read access to the
> hardware IA32_TSC_AUX MSR via RDTSCP, so we can allow it read access
> via the RDMSR instruction as well. If the guest doesn't support
> RDTSCP, intercept all accesses to the IA32_TSC_AUX MSR, so that kvm
> can synthesize a #GP.  (IA32_TSC_AUX exists iff RDTSCP is supported.)
> 
> Signed-off-by: Jim Mattson <jmattson@google.com>
> Reviewed-by: Marc Orr <marcorr@google.com>
> Reviewed-by: Peter Shier <pshier@google.com>
> Reviewed-by: Krish Sadhukhan <krish.sadhukhan@oracle.com>
> Reviewed-by: Sean Christopherson <sean.j.christopherson@intel.com>

Reviewed-by: Liran Alon <liran.alon@oracle.com>

-Liran

> 
> ---
> v1 -> v2: Rebased across vmx directory creation.
>          Modified commit message based on Sean's comments.
> 
> arch/x86/kvm/vmx/vmx.c | 4 ++++
> 1 file changed, 4 insertions(+)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index d175429c91b0..04a728976d96 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -4070,6 +4070,10 @@ static void vmx_compute_secondary_exec_control(struct vcpu_vmx *vmx)
> 
> 	if (vmx_rdtscp_supported()) {
> 		bool rdtscp_enabled = guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP);
> +
> +		vmx_set_intercept_for_msr(vmx->vmcs01.msr_bitmap, MSR_TSC_AUX,
> +					  MSR_TYPE_R, !rdtscp_enabled);
> +
> 		if (!rdtscp_enabled)
> 			exec_control &= ~SECONDARY_EXEC_RDTSCP;
> 
> -- 
> 2.24.0.393.g34dc348eaf-goog
>