linux-kernel - Re: [PATCH v4 1/3] x86/sev: Add new dump_rmp parameter to snp_leak

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Message-ID: <aMR3bRYEoR0eI6x7@google.com>
Date: Fri, 12 Sep 2025 12:41:33 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Tom Lendacky <thomas.lendacky@....com>
Cc: Ashish Kalra <Ashish.Kalra@....com>, tglx@...utronix.de, mingo@...hat.com, 
	bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com, 
	pbonzini@...hat.com, herbert@...dor.apana.org.au, nikunj@....com, 
	davem@...emloft.net, aik@....com, ardb@...nel.org, john.allen@....com, 
	michael.roth@....com, Neeraj.Upadhyay@....com, linux-kernel@...r.kernel.org, 
	kvm@...r.kernel.org, linux-crypto@...r.kernel.org
Subject: Re: [PATCH v4 1/3] x86/sev: Add new dump_rmp parameter to
 snp_leak_pages() API

On Fri, Sep 12, 2025, Tom Lendacky wrote:
> On 9/12/25 13:34, Sean Christopherson wrote:
> > But the below build failures show that they aren't dead code, which means that
> > kernels with CONFIG_KVM_AMD_SEV=n will silently (until something explodes) do the
> > wrong thing, because the stubs are hiding the missing dependencies.
> > 
> > arch/x86/boot/startup/sev-shared.c: In function ‘pvalidate_4k_page’:
> > arch/x86/boot/startup/sev-shared.c:820:17: error: implicit declaration of function ‘sev_evict_cache’ [-Wimplicit-function-declaration]
> >   820 |                 sev_evict_cache((void *)vaddr, 1);
> 
> Yeah, this one is on me. sev_evict_cache() is guest code and should be
> under the CONFIG_AMD_MEM_ENCRYPT #ifdef.
> 
> >       |                 ^~~~~~~~~~~~~~~
> >   AR      arch/x86/realmode/built-in.a
> > arch/x86/coco/sev/core.c: In function ‘pvalidate_pages’:
> > arch/x86/coco/sev/core.c:386:25: error: implicit declaration of function ‘sev_evict_cache’ [-Wimplicit-function-declaration]
> >   386 |                         sev_evict_cache(pfn_to_kaddr(e->gfn), e->pagesize ? 512 : 1);
> >       |                         ^~~~~~~~~~~~~~~
> > arch/x86/mm/mem_encrypt.c: In function ‘mem_encrypt_setup_arch’:
> > arch/x86/mm/mem_encrypt.c:112:17: error: implicit declaration of function ‘snp_fixup_e820_tables’ [-Wimplicit-function-declaration]
> >   112 |                 snp_fixup_e820_tables();
> 
> This function is only meant to be used if we're going to run SEV guests,
> so being guarded by CONFIG_KVM_AMD_SEV was on purpose. I'm just not sure
> why the stub didn't get used...  or did you remove them?

I removed all the stubs to see what would break (I was expecting nothing since
all of KVM's accesses are gated by CONFIG_KVM_AMD_SEV).