Re: [PATCH v2 3/4] KVM: SVM: Prevent writes to TSC MSR when Secure TSC is enabled

[Nikunj A Dadhania <nikunj@amd.com>]

Tom Lendacky <thomas.lendacky@amd.com> writes:

> On 2/10/25 03:22, Nikunj A Dadhania wrote:
>> Disallow writes to MSR_IA32_TSC for Secure TSC enabled SNP guests, as such
>> writes are not expected. Log the error and return #GP to the guest.
>
> Re-word this to make it a bit clearer about why this is needed. It is
> expected that the guest won't write to MSR_IA32_TSC or, if it does, it
> will ignore any writes to it and not exit to the HV. So this is catching
> the case where that behavior is not occurring.
>
Sure, will update.

>>
>> Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>
>> ---
>>  arch/x86/kvm/svm/svm.c | 11 +++++++++++
>>  1 file changed, 11 insertions(+)
>> 
>> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
>> index d7a0428aa2ae..929f35a2f542 100644
>> --- a/arch/x86/kvm/svm/svm.c
>> +++ b/arch/x86/kvm/svm/svm.c
>> @@ -3161,6 +3161,17 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
>>  
>>  		svm->tsc_aux = data;
>>  		break;
>> +	case MSR_IA32_TSC:
>> +		/*
>> +		 * If Secure TSC is enabled, KVM doesn't expect to receive
>> +		 * a VMEXIT for a TSC write, record the error and return a
>> +		 * #GP
>> +		 */
>> +		if (vcpu->arch.guest_state_protected && snp_secure_tsc_enabled(vcpu->kvm)) {
>
> Does it matter if the VMSA has already been encrypted? Can this just be
>
>   if (sev_snp_guest() && snp_secure_tsc_enabled(vcpu->kvm)) {
>
> ?
>

QEMU initializes the IA32_TSC MSR to zero resulting in the below
error if I use the above.

qemu-system-x86_64: error: failed to set MSR 0x10 to 0x0
qemu-system-x86_64: ../target/i386/kvm/kvm.c:3849: kvm_buf_set_msrs: Assertion `ret == cpu->kvm_msr_buf->nmsrs' failed.

Once the guest state is protected, we do not expect any writes from VMM.

Regards,
Nikunj