Re: [PATCH 1/3] mm: userfaultfd: add new UFFDIO_SIGBUS ioctl

[Peter Xu <peterx@redhat.com>]

On Thu, May 11, 2023 at 03:00:09PM -0700, James Houghton wrote:
> On Thu, May 11, 2023 at 11:24 AM Axel Rasmussen
> <axelrasmussen@google.com> wrote:
> >
> > So the basic way to use this new feature is:
> >
> > - On the new host, the guest's memory is registered with userfaultfd, in
> >   either MISSING or MINOR mode (doesn't really matter for this purpose).
> > - On any first access, we get a userfaultfd event. At this point we can
> >   communicate with the old host to find out if the page was poisoned.
> > - If so, we can respond with a UFFDIO_SIGBUS - this places a swap marker
> >   so any future accesses will SIGBUS. Because the pte is now "present",
> >   future accesses won't generate more userfaultfd events, they'll just
> >   SIGBUS directly.
> 
> I want to clarify the SIGBUS mechanism here when KVM is involved,
> keeping in mind that we need to be able to inject an MCE into the
> guest for this to be useful.
> 
> 1. vCPU gets an EPT violation --> KVM attempts GUP.
> 2. GUP finds a PTE_MARKER_UFFD_SIGBUS and returns VM_FAULT_SIGBUS.
> 3. KVM finds that GUP failed and returns -EFAULT.
> 
> This is different than if GUP found poison, in which case KVM will
> actually queue up a SIGBUS *containing the address of the fault*, and
> userspace can use it to inject an appropriate MCE into the guest. With
> UFFDIO_SIGBUS, we are missing the address!
> 
> I see three options:
> 1. Make KVM_RUN queue up a signal for any VM_FAULT_SIGBUS. I think
> this is pointless.
> 2. Don't have UFFDIO_SIGBUS install a PTE entry, but instead have a
> UFFDIO_WAKE_MODE_SIGBUS, where upon waking, we return VM_FAULT_SIGBUS
> instead of VM_FAULT_RETRY. We will keep getting userfaults on repeated
> accesses, just like how we get repeated signals for real poison.
> 3. Use this in conjunction with the additional KVM EFAULT info that
> Anish proposed (the first part of [1]).
> 
> I think option 3 is fine. :)

Or... option 4) just to use either MADV_HWPOISON or hwpoison-inject? :)

Besides what James mentioned on "missing addr", I didn't quickly see what's
the major difference comparing to the old hwpoison injection methods even
without the addr requirement. If we want the addr for MCE then it's more of
a question to ask.

I also didn't quickly see why for whatever new way to inject a pte error we
need to have it registered with uffd.  Could it be something like
MADV_PGERR (even if MADV_HWPOISON won't suffice) so you can inject even
without an userfault context (but still usable when uffd registered)?

And it'll be alawys nice to have a cover letter too (if there'll be a new
version) explaining the bits.

Thanks,

-- 
Peter Xu