On 2/11/25 02:24, Nikunj A Dadhania wrote:
> Tom Lendacky <thomas.lendacky@xxxxxxx> 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@xxxxxxx>
>>> ---
>>> 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.
Should KVM be doing anything related to MSR_IA32_TSC for a Secure TSC
guest, even handling this Qemu write? That Qemu write takes it through
the kvm_synchronize_tsc() path, does it need to? I'm just wondering if
the Secure TSC HV support needs more handling of MSR_IA32_TSC (in both
set and get) than what's here. Thoughts?
Thanks,
Tom
>
> Once the guest state is protected, we do not expect any writes from VMM.
>
> Regards,
> Nikunj
