Re: [PATCH v12 34/84] KVM: Add a helper to lookup a pfn without grabbing a reference

Paolo Bonzini
2024-07-31T10:11:57Z

On 7/30/24 22:15, Sean Christopherson wrote:
> On Tue, Jul 30, 2024, Paolo Bonzini wrote:
>> On 7/27/24 01:51, Sean Christopherson wrote:
>>> Add a kvm_follow_pfn() wrapper, kvm_lookup_pfn(), to allow looking up a
>>> gfn=>pfn mapping without the caller getting a reference to any underlying
>>> page. The API will be used in flows that want to know if a gfn points at
>>> a valid pfn, but don't actually need to do anything with the pfn.
>>
>> Can you rename the function kvm_gfn_has_pfn(), or kvm_gfn_can_be_mapped(),
>> and make it return a bool?
>
> Heh, sure. I initially planned on having it return a bool, but I couldn't figure
> out a name, mainly because the kernel's pfn_valid() makes things like
> kvm_gfn_has_valid_pfn() confusing/misleading :-(
>
>> (As an aside, I wonder if reexecute_instruction() could just use
>> kvm_is_error_hva(kvm_vcpu_gfn_to_hva(vcpu, gpa_to_gfn(gpa)) instead of going
>> all the way to a pfn. But it's ok to be more restrictive).
>
> Heh #2, I wondered the same thing. I think it would work? Verifying that there's
> a usable pfn also protects against retrying an access that hit -EHWPOISON, but I'm
> prety sure that would require a rare race, and I don't think it could result in
> the guest being put into an infinite loop.

Indeed, and even the check in kvm_alloc_apic_access_page() is totally
useless. The page can go away at any time between the call and
vmx_set_apic_access_page_addr() or, for AMD, the #NPF on
APIC_DEFAULT_PHYS_BASE.

Yes, it's verifying that the system isn't under extreme memory pressure,
but in practice a 4K get_user_pages is never going to fail, it's just
going to cause something else to be swapped. I'd just get rid of both
of them, so there's no need for kvm_lookup_pfn().

Paolo