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

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Message-ID: <87h7usbkhq.fsf@vitty.brq.redhat.com>
Date:   Tue, 30 Jun 2020 18:12:49 +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,
        virtio-fs@...hat.com, pbonzini@...hat.com,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] kvm,x86: Exit to user space in case of page fault error

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

> On Tue, Jun 30, 2020 at 05:43:54PM +0200, Vitaly Kuznetsov wrote:
>> Vivek Goyal <vgoyal@...hat.com> writes:
>> 
>> > On Tue, Jun 30, 2020 at 05:13:54PM +0200, Vitaly Kuznetsov wrote:
>> >> 
>> >> > - If you retry in kernel, we will change the context completely that
>> >> >   who was trying to access the gfn in question. We want to retain
>> >> >   the real context and retain information who was trying to access
>> >> >   gfn in question.
>> >> 
>> >> (Just so I understand the idea better) does the guest context matter to
>> >> the host? Or, more specifically, are we going to do anything besides
>> >> get_user_pages() which will actually analyze who triggered the access
>> >> *in the guest*?
>> >
>> > When we exit to user space, qemu prints bunch of register state. I am
>> > wondering what does that state represent. Does some of that traces
>> > back to the process which was trying to access that hva? I don't
>> > know.
>> 
>> We can get the full CPU state when the fault happens if we need to but
>> generally we are not analyzing it. I can imagine looking at CPL, for
>> example, but trying to distinguish guest's 'process A' from 'process B'
>> may not be simple.
>> 
>> >
>> > I think keeping a cache of error gfns might not be too bad from
>> > implemetation point of view. I will give it a try and see how
>> > bad does it look.
>> 
>> Right; I'm only worried about the fact that every cache (or hash) has a
>> limited size and under certain curcumstances we may overflow it. When an
>> overflow happens, we will follow the APF path again and this can go over
>> and over. Maybe we can punch a hole in EPT/NPT making the PFN reserved/
>> not-present so when the guest tries to access it again we trap the
>> access in KVM and, if the error persists, don't follow the APF path?
>
> Just to make sure I'm somewhat keeping track, is the problem we're trying to
> solve that the guest may not immediately retry the "bad" GPA and so KVM may
> not detect that the async #PF already came back as -EFAULT or whatever? 

Yes. In Vivek's patch there's a single 'error_gfn' per vCPU which serves
as an indicator whether to follow APF path or not.

-- 
Vitaly