linux-kernel - Re: [PATCH v12 00/84] KVM: Stop grabbing references to PFNMAP'd pages

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Message-ID: <419ea6ce-83ca-413e-936c-1935e2c51497@redhat.com>
Date: Tue, 30 Jul 2024 13:52:15 +0200
From: Paolo Bonzini <pbonzini@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>, 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>
Cc: 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 00/84] KVM: Stop grabbing references to PFNMAP'd pages

On 7/27/24 01:51, Sean Christopherson wrote:
> arm64 folks, the first two patches are bug fixes, but I have very low
> confidence that they are correct and/or desirable.  If they are more or
> less correct, I can post them separately if that'd make life easier.  I
> included them here to avoid conflicts, and because I'm pretty sure how
> KVM deals with MTE tags vs. dirty logging will impact what APIs KVM needs
> to provide to arch code.
> 
> On to the series...  The TL;DR is that I would like to get input on two
> things:
> 
>   1. Marking folios dirty/accessed only on the intial stage-2 page fault
>   2. The new APIs for faulting, prefetching, and doing "lookups" on pfns

Wow!

Splitting out prefetching makes a lot of sense, as it's the only one 
with npages > 1 and it doesn't need all the complexity of hva_to_pfn().

I've left a comment on the lookup API, which is probably the only one 
that can be simplified further.

The faulting API looks good as a first iteration.  Code-wise, 
kvm_resolve_pfn() is probably unnecessary at the end of the series but I 
can see why you had to restrain yourself and declare it done. :)

An interesting evolution of the API could be to pass a struct 
kvm_follow_pfn pointer to {,__}kvm_faultin_pfn() and __gfn_to_page() 
(the "constructors"); and on the other side to 
kvm_release_faultin_page() and kvm_release_page_*().  The struct 
kvm_follow_pfn could be embedded in the (x86) kvm_page_fault and 
(generic) kvm_host_map structs.  But certainly not as part of this 
already huge work.

Paolo