Re: [PATCH v3 00/22] Improve scalability of KVM + userfaultfd live migration via annotated memory faults.

[Oliver Upton <oliver.upton@linux.dev>]

On Tue, May 23, 2023 at 10:49:04AM -0700, Anish Moorthy wrote:
> On Wed, May 10, 2023 at 4:44 PM Anish Moorthy <amoorthy@google.com> wrote:
> >
> > On Wed, May 10, 2023 at 3:35 PM Sean Christopherson <seanjc@google.com> wrote:
> > >
> > > Yeah, when I speed read the series, several of the conversions stood out as being
> > > "wrong".  My (potentially unstated) idea was that KVM would only signal
> > > KVM_EXIT_MEMORY_FAULT when the -EFAULT could be traced back to a user access,
> > > i.e. when the fault _might_ be resolvable by userspace.
> >
> > Sean, besides direct_map which other patches did you notice as needing
> > to be dropped/marked as unrecoverable errors?
> 
> I tried going through on my own to try and identify the incorrect
> annotations: here's my read.
> 
> Correct (or can easily be corrected)
> -----------------------------------------------
> - user_mem_abort
>   Incorrect as is: the annotations in patch 19 are incorrect, as they
> cover an error-on-no-slot case and one more I don't fully understand:

That other case is a wart we endearingly refer to as MTE (Memory Tagging
Extension). You theoretically _could_ pop out an annotated exit here, as
userspace likely messed up the mapping (like PROT_MTE missing).

But I'm perfectly happy letting someone complain about it before we go
out of our way to annotate that one. So feel free to drop.

-- 
Thanks,
Oliver