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

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Message-ID: <ZqlqpjO0TiWnOEqx@google.com>
Date: Tue, 30 Jul 2024 15:35:18 -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 00/84] KVM: Stop grabbing references to PFNMAP'd pages

On Tue, Jul 30, 2024, Paolo Bonzini wrote:
> 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.

For kvm_faultin_pfn(), my hope/dream is to make kvm_page_fault a common struct,
with an arch member (a la kvm_vcpu), and get to something like:

  static int arch_page_fault_handler(...)
  {
	struct kvm_page_fault fault = {
		<const common stuff>,

		.arch.xxx = <arch stuff>,
	};

	<arch code>


	r = kvm_faultin_pfn();
	
	...
  }

In theory, that would allow moving the kvm->mmu_invalidate_seq handling, memslot
lookup, etc. into kvm_faultin_pfn(), or maybe another helper that is invoked to
setup the fault structure.  I.e. it would give us a way to drive convergence for
at least some of the fault handling logic, without having to tackle gory arch
details, at least not right away.