Re: [PATCH v3 8/9] target/i386: SEV: Add support for setting TSC frequency for Secure TSC

["Daniel P. Berrangé" <berrange@redhat.com>]

On Fri, Nov 07, 2025 at 02:21:24PM +0530, Naveen N Rao wrote:
> On Thu, Nov 06, 2025 at 01:09:37PM +0100, Markus Armbruster wrote:
> > Pardon my ignorance...
> > 
> > "Naveen N Rao (AMD)" <naveen@kernel.org> 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'
> 
> > 
> > >                 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).

The '-cpu' arg is global to the VM, so even though the ioctl is per-VCPU,
a single '-cpu ...,tsc-frequency=NNN' argument applies universally to all
the vCPUs in regular guests. 

> This was Tom's question too (see v2): is there any way to re-use 
> 'tsc-frequency' specified with '-cpu' for Secure TSC.

I see no reason why we can't simply use the existing '-cpu tsc-frequency'
value. Fetch the CPU 0 object and query its "tsc-frequency" property,
and just assume all non-0 CPUs have the same tsc-frequency, since we
don't provide a way to set it differently per-CPU IIUC.


With regards,
Daniel
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