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

[Peter Xu]

On Wed, May 17, 2023 at 06:12:33PM -0400, Peter Xu wrote:
> 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@xxxxxxxxxx> 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? :)

I just remember Axel mentioned this in the commit message, and just in case
this is why option 4) was ruled out:

        They expect that once poisoned, pages can never become
        "un-poisoned". So, when we live migrate the VM, we need to preserve
        the poisoned status of these pages.

Just to supplement on this point: we do have unpoison (echoing to
"debug/hwpoison/hwpoison_unpoison"), or am I wrong?

> 
> 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

-- 
Peter Xu