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@xxxxxxxxx>
>>>
>>> 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.
