Re: [PATCH 3/4] crypto/ccp: support setting RAPL_DIS in SNP_INIT_EX

[Sean Christopherson <seanjc@google.com>]

On Mon, Feb 23, 2026, Tycho Andersen wrote:
> On Mon, Feb 23, 2026 at 08:40:19AM -0800, Sean Christopherson wrote:
> > On Mon, Feb 23, 2026, Tycho Andersen wrote:
> > > From: "Tycho Andersen (AMD)" <tycho@kernel.org>
> > > 
> > > The kernel allows setting the RAPL_DIS policy bit, but had no way to set
> > 
> > Please actually say what RAPL_DIS is and does, and explain why this is the
> > correct approach.  I genuinely have no idea what the impact of this patch is,
> > (beyond disabling something, obviously).
> 
> Sure, the easiest thing is probably to quote the firmware PDF:
> 
>     Some processors support the Running Average Power Limit (RAPL)
>     feature which provides information about power utilization of
>     software. RAPL can be disabled using the RAPL_DIS flag in
>     SNP_INIT_EX to disable RAPL while SNP firmware is in the INIT
>     state. Guests may require that RAPL is disabled by using the
>     POLICY.RAPL_DIS guest policy flag.

Ah, I assume this about disabling RAPL to mitigate a potential side channel?  If
so, please call that out in the changelog.

And does this disable RAPL for _everything_?  Or does it just disable RAPL for
SNP VMs?  If it's the former, then burying this in drivers/crypto/ccp/sev-dev.c
feels wrong.