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Date: Wed, 25 Feb 2026 11:05:57 -0700
From: Tycho Andersen <tycho@kernel.org>
To: Sean Christopherson <seanjc@google.com>
Cc: Ashish Kalra <ashish.kalra@amd.com>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	John Allen <john.allen@amd.com>,
	Herbert Xu <herbert@gondor.apana.org.au>,
	Paolo Bonzini <pbonzini@redhat.com>, Shuah Khan <shuah@kernel.org>,
	"David S. Miller" <davem@davemloft.net>,
	linux-crypto@vger.kernel.org, linux-kernel@vger.kernel.org,
	kvm@vger.kernel.org, linux-kselftest@vger.kernel.org
Subject: Re: [PATCH 3/4] crypto/ccp: support setting RAPL_DIS in SNP_INIT_EX
Message-ID: <aZ86BZWi-GLiHvmt@tycho.pizza>
References: <20260223162900.772669-1-tycho@kernel.org>
 <20260223162900.772669-4-tycho@kernel.org>
 <aZyC89v9JAVEPeLt@google.com>
 <aZzRVXp_E3cMcgtX@tycho.pizza>
 <aZ3k01UObX03Sv-n@google.com>
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In-Reply-To: <aZ3k01UObX03Sv-n@google.com>

On Tue, Feb 24, 2026 at 09:50:11AM -0800, Sean Christopherson wrote:
> 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.

Presumably you're right on both counts, but I've asked our firmware
team to clarify exactly what happens.

I guess that means it should be kvm-amd.rapl_disable?

Thanks,

Tycho