On 8/20/2025 6:31 PM, Sean Christopherson wrote:
> On Wed, Aug 20, 2025, Nikunj A. Dadhania wrote:
>>
>>
>> On 8/20/2025 5:18 AM, Sean Christopherson wrote:
>>> From: Nikunj A Dadhania <nikunj@xxxxxxx>
>>>
>>> @@ -2195,6 +2206,12 @@ static int snp_launch_start(struct kvm *kvm, struct kvm_sev_cmd *argp)
>>>
>>> start.gctx_paddr = __psp_pa(sev->snp_context);
>>> start.policy = params.policy;
>>> +
>>> + if (snp_is_secure_tsc_enabled(kvm)) {
>>> + WARN_ON_ONCE(!kvm->arch.default_tsc_khz);
>>
>> Any particular reason to drop the the following change:
>>
>> + if (WARN_ON(!kvm->arch.default_tsc_khz)) {
>> + rc = -EINVAL;
>> + goto e_free_context;
>> + }
>
> Based on this conversation[*], both Kai and I expected KVM to let firmware deal
> with the should-be-impossible situation.
>
> On Tue, Jul 8, 2025 at 9:15 PM Nikunj A. Dadhania <nikunj@xxxxxxx> wrote:
> > On 7/8/2025 8:04 PM, Sean Christopherson wrote:
> > > On Tue, Jul 08, 2025, Kai Huang wrote:
> > >>>> Even some bug results in the default_tsc_khz being 0, will the
> > >>>> SNP_LAUNCH_START command catch this and return error?
> > >>>
> > >>> No, that is an invalid configuration, desired_tsc_khz is set to 0 when
> > >>> SecureTSC is disabled. If SecureTSC is enabled, desired_tsc_khz should
> > >>> have correct value.
> > >>
> > >> So it's an invalid configuration that when Secure TSC is enabled and
> > >> desired_tsc_khz is 0. Assuming the SNP_LAUNCH_START will return an error
> > >> if such configuration is used, wouldn't it be simpler if you remove the
> > >> above check and depend on the SNP_LAUNCH_START command to catch the
> > >> invalid configuration?
> > >
> > > Support for secure TSC should depend on tsc_khz being non-zero. That way it'll
> > > be impossible for arch.default_tsc_khz to be zero at runtime. Then KVM can WARN
> > > on arch.default_tsc_khz being zero during SNP_LAUNCH_START.
> >
> > Sure.
>
> https://lore.kernel.org/all/c327df02-c2eb-41e7-9402-5a16aa211265@xxxxxxx
>
>>
>> As this is an unsupported configuration as per the SEV SNP Firmware ABI Specification:
>
> Right, but what happens if KVM manages to pass in '0' for the frequency? Does
> SNP_LAUNCH_START fail?
SNP_LAUNCH_START succeeds, and the guest kernel starts and panics during early boot [*]
> If so, bailing from KVM doesn't seem to add any value.
As firmware does not bail out, I had kept this check.
RegardsNikunjhttps://lore.kernel.org/all/afcf9a0b-7450-4df7-a21b-80b56264fc15@xxxxxxx
