linux-kernel - Re: [PATCH v12 34/84] KVM: Add a helper to lookup a pfn without grabbing a reference

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Message-ID: <ZqlJxJyOdsR206Zc@google.com>
Date: Tue, 30 Jul 2024 13:15:00 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: Marc Zyngier <maz@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>, 
	Tianrui Zhao <zhaotianrui@...ngson.cn>, Bibo Mao <maobibo@...ngson.cn>, 
	Huacai Chen <chenhuacai@...nel.org>, Michael Ellerman <mpe@...erman.id.au>, 
	Anup Patel <anup@...infault.org>, Paul Walmsley <paul.walmsley@...ive.com>, 
	Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>, 
	Christian Borntraeger <borntraeger@...ux.ibm.com>, Janosch Frank <frankja@...ux.ibm.com>, 
	Claudio Imbrenda <imbrenda@...ux.ibm.com>, kvm@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev, 
	loongarch@...ts.linux.dev, linux-mips@...r.kernel.org, 
	linuxppc-dev@...ts.ozlabs.org, kvm-riscv@...ts.infradead.org, 
	linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	David Matlack <dmatlack@...gle.com>, David Stevens <stevensd@...omium.org>
Subject: Re: [PATCH v12 34/84] KVM: Add a helper to lookup a pfn without
 grabbing a reference

On Tue, Jul 30, 2024, Paolo Bonzini wrote:
> On 7/27/24 01:51, Sean Christopherson wrote:
> > Add a kvm_follow_pfn() wrapper, kvm_lookup_pfn(), to allow looking up a
> > gfn=>pfn mapping without the caller getting a reference to any underlying
> > page.  The API will be used in flows that want to know if a gfn points at
> > a valid pfn, but don't actually need to do anything with the pfn.
> 
> Can you rename the function kvm_gfn_has_pfn(), or kvm_gfn_can_be_mapped(),
> and make it return a bool?

Heh, sure.  I initially planned on having it return a bool, but I couldn't figure
out a name, mainly because the kernel's pfn_valid() makes things like
kvm_gfn_has_valid_pfn() confusing/misleading :-(

> (As an aside, I wonder if reexecute_instruction() could just use
> kvm_is_error_hva(kvm_vcpu_gfn_to_hva(vcpu, gpa_to_gfn(gpa)) instead of going
> all the way to a pfn.  But it's ok to be more restrictive).

Heh #2, I wondered the same thing.  I think it would work?  Verifying that there's
a usable pfn also protects against retrying an access that hit -EHWPOISON, but I'm
prety sure that would require a rare race, and I don't think it could result in
the guest being put into an infinite loop.