Re: [PATCH 3/3] KVM: arm64: Do a KVM_EXIT_MEMORY_FAULT when stage-2 fault handler EFAULTs

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

On Tue, Aug 06, 2024 at 11:44:39AM -0700, Anish Moorthy wrote:
> On Mon, Aug 5, 2024 at 4:02 PM Oliver Upton <oliver.upton@linux.dev> wrote:
> >
> > On Fri, Aug 02, 2024 at 10:40:31PM +0000, Anish Moorthy wrote:
> > > Right now userspace just gets a bare EFAULT when the stage-2 fault
> > > handler fails to fault in the relevant page. Set up a memory fault exit
> > > when this happens, which at the very least eases debugging and might
> > > also let userspace decide on/take some specific action other than
> > > crashing the VM.
> >
> > There are several other 'bare' EFAULTs remaining (unexpected fault
> > context, failed vma_lookup(), nested PTW), so the patch doesn't exactly
> > match the shortlog.
> >
> > Is there a reason why those are unaddressed? In any case, it doesn't
> > hurt to be unambiguous in the shortlog if we're only focused on this single
> > error condition, e.g.
> >
> >   KVM: arm64: Do a memory fault exit if __gfn_to_pfn_memslot() fails
> 
> Ah, right- forgot to address this before I sent it out.
> 
> Basically: those cases you mention (besides MTE, where it seems simple
> enough to add an annotation) happen before vma_pagesize is calculated,

If the motivation is to add additional information for debugging
unexpected KVM/VMM behavior then this really ought to be addressed. You
could fall back to PAGE_SIZE, or better yet just don't report a size
whatsoever (size = 0) if it cannot be reliably determined.

Userspace probably only cares about logging @flags and @gpa before
killing the VM.

-- 
Thanks,
Oliver