Re: [PATCH v2 3/4] KVM: SEV: Add the kvm-amd.rapl_disable module parameter

[Tycho Andersen <tycho@kernel.org>]

On Mon, Apr 27, 2026 at 02:20:10PM -0700, Sean Christopherson wrote:
> On Mon, Apr 27, 2026, Tycho Andersen wrote:
> > From: "Tycho Andersen (AMD)" <tycho@kernel.org>
> > 
> > Add a user-visible way to set the RAPL_DIS bit for SNP init.
> > 
> > Since setting RAPL_DIS affects the whole system, put the module parameter
> > in kvm_amd instead of in the CCP driver to hopefully make it more obvious
> > to admins.
> > 
> > Signed-off-by: Tycho Andersen (AMD) <tycho@kernel.org>
> > ---
> >  Documentation/admin-guide/kernel-parameters.txt | 5 +++++
> >  arch/x86/kvm/svm/sev.c                          | 8 ++++++++
> >  2 files changed, 13 insertions(+)
> > 
> > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> > index 4d0f545fb3ec..2b50eed8664c 100644
> > --- a/Documentation/admin-guide/kernel-parameters.txt
> > +++ b/Documentation/admin-guide/kernel-parameters.txt
> > @@ -3207,6 +3207,11 @@ Kernel parameters
> >  			max_snp_asid == min_sev_asid-1, will effectively make
> >  			SEV-ES unusable.
> >  
> > +	kvm-amd.rapl_disable=	[KVM,AMD] Whether to disable RAPL
> > +			(Running Average Power Limit) when initializing the SNP
> > +			firmware. This disables the counters for the entire system until an
> > +			SNP shutdown command is issued.
> 
> I'm pretty sure I said this earlier: KVM absolutely should not be able to disable
> RAPL for the entire system.  That needs to be a power management thing.

You definitely noted "not CCP", I don't think I quite understood what
that meant though:
https://lore.kernel.org/all/aZ86BZWi-GLiHvmt@tycho.pizza/

I'm a little worried that putting it in power management will generate
some weird dependencies, or weakref symbols that can't change things
if they are loaded independently of kvm_amd or something. But let me
see what I can come up with.

> KVM then needs to communicate (and enforce?) the policy to
> userspace.

KVM doesn't need to enforce anything, the SEV firmware will generate a
launch error for policy violation if it's not supported.

For communicating to userspace if it's not a kvm module parameter, one
option is to mask it off in sev_get_snp_supported_policy() if it was
initialized without the support. Then it'll be visible via
KVM_X86_SNP_POLICY_BITS.

Thanks,

Tycho