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

[Sean Christopherson]

On Tue, Jun 30, 2020 at 06:12:49PM +0200, Vitaly Kuznetsov wrote:
> Sean Christopherson <sean.j.christopherson@xxxxxxxxx> writes:
> 
> > On Tue, Jun 30, 2020 at 05:43:54PM +0200, Vitaly Kuznetsov wrote:
> >> Vivek Goyal <vgoyal@xxxxxxxxxx> 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.

A thought along the lines of your "punch a hole in the page tables" idea
would be to invalidate the SPTE (in the unlikely case it's present but not
writable) and tagging it as being invalid for async #PF.  E.g. for !EPT,
there are 63 bits available for metadata.  For EPT, there's a measly 60,
assuming we want to avoid using SUPPRESS_VE.  The fully !present case would
be straightforward, but the !writable case would require extra work,
especially for shadow paging.

With the SPTE tagged, it'd "just" be a matter of hooking into the page fault
paths to detect the flag and disable async #PF.  For TDP that's not too bad,
e.g. pass in a flag to fast_page_fault() and propagate it to try_async_pf().
Not sure how to handle shadow paging, that code makes my head hurt just
looking at it.

It'd require tweaking is_shadow_present_pte() to be more precise, but that's
probably a good thing, and peanuts compared to handling the faults.