linux-kernel - Re: [PATCH v4] kvm,x86: Exit to user space in case page fault error

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Message-ID: <87y2kj71gj.fsf@vitty.brq.redhat.com>
Date:   Tue, 06 Oct 2020 18:39:56 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Vivek Goyal <vgoyal@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        virtio-fs-list <virtio-fs@...hat.com>, pbonzini@...hat.com
Subject: Re: [PATCH v4] kvm,x86: Exit to user space in case page fault error

Sean Christopherson <sean.j.christopherson@...el.com> writes:

> On Tue, Oct 06, 2020 at 05:24:54PM +0200, Vitaly Kuznetsov wrote:
>> Vivek Goyal <vgoyal@...hat.com> writes:
>> > So you will have to report token (along with -EFAULT) to user space. So this
>> > is basically the 3rd proposal which is extension of kvm API and will
>> > report say HVA/GFN also to user space along with -EFAULT.
>> 
>> Right, I meant to say that guest kernel has full register state of the
>> userspace process which caused APF to get queued and instead of trying
>> to extract it in KVM and pass to userspace in case of a (later) failure
>> we limit KVM api change to contain token or GFN only and somehow keep
>> the rest in the guest. This should help with TDX/SEV-ES.
>
> Whatever gets reported to userspace should be identical with and without
> async page faults, i.e. it definitely shouldn't have token information.
>

Oh, right, when the error gets reported synchronously guest's kernel is
not yet aware of the issue so it won't be possible to find anything in
its kdump if userspace decides to crash it immediately. The register
state (if available) will be actual though.

> Note, TDX doesn't allow injection exceptions, so reflecting a #PF back
> into the guest is not an option.  

Not even #MC? So sad :-)

> Nor do I think that's "correct" behavior (see everyone's objections to
> using #PF for APF fixed).  I.e. the event should probably be an IRQ.

I recall Paolo objected against making APF 'page not present' into in
interrupt as it will require some very special handling to make sure it
gets injected (and handled) immediately but I'm not really sure how big
the hack is going to be, maybe in the light of TDX/SEV-ES it's worth a
try.

-- 
Vitaly