Naveen N Rao <[email protected]> writes: > On Thu, Nov 06, 2025 at 01:09:37PM +0100, Markus Armbruster wrote: >> Pardon my ignorance... >> >> "Naveen N Rao (AMD)" <[email protected]> writes: >> >> > Add support for configuring the TSC frequency when Secure TSC is enabled >> > in SEV-SNP guests through a new "tsc-frequency" property on SEV-SNP >> > guest objects, similar to the vCPU-specific property used by regular >> > guests and TDX. >> >> Which property exactly? > > Same name: tsc-frequency specified with '-cpu'
Thanks. It's x86_64-cpu property tsc-frequency. >> >> > A new property is needed since SEV-SNP guests require >> > the TSC frequency to be specified during early SNP_LAUNCH_START command >> > before any vCPUs are created. >> >> Sounds awkward. >> >> Do the two properties set the same thing at different times? > > Yes. For regular guests, TSC frequency is set using a vCPU ioctl. > However, TDX and SEV-SNP (with Secure TSC) require the TSC frequency to > be set as a VM property (there is a VM ioctl for this purpose). > > This was Tom's question too (see v2): is there any way to re-use > 'tsc-frequency' specified with '-cpu' for Secure TSC. Hmm, let's see whether I can guess how this stuff works. Please correct my misunderstandings. When machine property confidential-guest-support is null, it's a regular guest. If it points to a sev-guest object, it's SEV. If it points to a sev-snp-guest object, it's SEV-SNP. If it points to a tdx-guest object, it's TDX. Normally, the TSC frequency is specified with x86_64-cpu property tsc-frequency. Can different CPUs have different frequencies? In certain cases (SEV-SNP or TDX guest with Secure TSC), tsc-frequency needs to be configured before any CPUs are created. You're implementing this for SEV-SNP, and you chose to create a sev-snp property tsc-frequency for this. What happens when I enable Secure TSC with sev-snp property "secure-tsc": true, but don't set property tsc-frequency? What happens when I do set it, and then also set the CPU property? To the same frequency? To a different frequency? >> > The user-provided TSC frequency is set through KVM_SET_TSC_KHZ before >> > issuing KVM_SEV_SNP_LAUNCH_START. >> > >> > Attempts to set TSC frequency on both the SEV_SNP object and the cpu >> > object result in an error from KVM (on the vCPU ioctl), so do not add >> > separate checks for the same. >> > >> > Sample command-line: >> > -machine q35,confidential-guest-support=sev0 \ >> > -object >> > sev-snp-guest,id=sev0,cbitpos=51,reduced-phys-bits=1,secure-tsc=on,tsc-frequency=2500000000 >> > >> > Co-developed-by: Ketan Chaturvedi <[email protected]> >> > Signed-off-by: Ketan Chaturvedi <[email protected]> >> > Co-developed-by: Nikunj A Dadhania <[email protected]> >> > Signed-off-by: Nikunj A Dadhania <[email protected]> >> > Signed-off-by: Naveen N Rao (AMD) <[email protected]> >> >> [...] >> >> > diff --git a/qapi/qom.json b/qapi/qom.json >> > index c7dd2dd1b095..5daaf065b6b7 100644 >> > --- a/qapi/qom.json >> > +++ b/qapi/qom.json >> > @@ -1104,6 +1104,9 @@ >> > # @secure-tsc: enable Secure TSC >> > # (default: false) (since 10.2) >> > # >> > +# @tsc-frequency: set secure TSC frequency. Only valid if Secure TSC >> > +# is enabled (default: zero) (since 10.2) >> >> Is this likely to remain the only property that's only valied when >> @secure-tsc is true? > > At this stage, yes. I am not aware of anything else that is specific to > Secure TSC. Alright, this makes "only valid if" reasonable.
