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: <a0418ced-260b-4b14-9124-c94ce46a0944@redhat.com>
Date: Wed, 31 Jul 2024 12:11:17 +0200
From: Paolo Bonzini <pbonzini@...hat.com>
To: Sean Christopherson <seanjc@...gle.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 7/30/24 22:15, Sean Christopherson wrote:
> 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.

Indeed, and even the check in kvm_alloc_apic_access_page() is totally 
useless.  The page can go away at any time between the call and 
vmx_set_apic_access_page_addr() or, for AMD, the #NPF on 
APIC_DEFAULT_PHYS_BASE.

Yes, it's verifying that the system isn't under extreme memory pressure, 
but in practice a 4K get_user_pages is never going to fail, it's just 
going to cause something else to be swapped.  I'd just get rid of both 
of them, so there's no need for kvm_lookup_pfn().

Paolo