linux-kernel - Re: [PATCH 04/16] KVM: x86/mmu: Pass full 64-bit error code when handling page faults

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Message-ID: <Zd9d2n3k8c5pTJto@google.com>
Date: Wed, 28 Feb 2024 08:22:50 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Dongli Zhang <dongli.zhang@...cle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Yan Zhao <yan.y.zhao@...el.com>, Isaku Yamahata <isaku.yamahata@...el.com>, 
	Michael Roth <michael.roth@....com>, Yu Zhang <yu.c.zhang@...ux.intel.com>, 
	Chao Peng <chao.p.peng@...ux.intel.com>, Fuad Tabba <tabba@...gle.com>, 
	David Matlack <dmatlack@...gle.com>
Subject: Re: [PATCH 04/16] KVM: x86/mmu: Pass full 64-bit error code when
 handling page faults

On Tue, Feb 27, 2024, Dongli Zhang wrote:
> 
> 
> On 2/27/24 18:41, Sean Christopherson wrote:
> > From: Isaku Yamahata <isaku.yamahata@...el.com>
> > 
> > Plumb the full 64-bit error code throughout the page fault handling code
> > so that KVM can use the upper 32 bits, e.g. SNP's PFERR_GUEST_ENC_MASK
> > will be used to determine whether or not a fault is private vs. shared.
> > 
> > Note, passing the 64-bit error code to FNAME(walk_addr)() does NOT change
> > the behavior of permission_fault() when invoked in the page fault path, as
> > KVM explicitly clears PFERR_IMPLICIT_ACCESS in kvm_mmu_page_fault().
> 
> May this lead to a WARN_ON_ONCE?
> 
> 5843 int noinline kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
> u64 error_code,
> 5844                        void *insn, int insn_len)
> 5845 {
> ... ...
> 5856          */
> 5857         if (WARN_ON_ONCE(error_code & PFERR_IMPLICIT_ACCESS))
> 5858                 error_code &= ~PFERR_IMPLICIT_ACCESS;

Nope, it shouldn't.  PFERR_IMPLICIT_ACCESS is a synthetic, KVM-defined flag, and
should never be in the error code passed to kvm_mmu_page_fault().  If the WARN
fires, it means hardware (specifically, AMD CPUs for #NPF) has started using the
bit for something, and that we need to update KVM to use a different bit.

> > Continue passing '0' from the async #PF worker, as guest_memfd() and thus
> 
> :s/guest_memfd()/guest_memfd/ ?

I've been styling it as guest_memfd() to make it look like a syscall, e.g. like
memfd_create(), when I'm talking about a file that was created by userspace, as
opposed to GUEST_MEMFD when I'm talking about the ioctl() itself.