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

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Date: Thu, 29 Feb 2024 05:32:48 -0800
From: Dongli Zhang <dongli.zhang@...cle.com>
To: Sean Christopherson <seanjc@...gle.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 2/28/24 08:22, Sean Christopherson wrote:
> 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.

Thank you very much for the explanation.

I see it is impossible to have PFERR_IMPLICIT_ACCESS set here, unless there is
AMD hardware issue or Intel page fault handler morphs the error_code erroneously.

I meant the above commit message confused me when I was reading it. E.g., how
about something like:

"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 it
should never be in the error code because ...."


Thank you very much!

Dongli Zhang

> 
>>> 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.