Re: [RFC PATCH v12 07/33] KVM: Add KVM_EXIT_MEMORY_FAULT exit to report faults to userspace

[Anish Moorthy]

On Mon, Oct 2, 2023 at 6:43 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> > - I should go drop the patches annotating kvm_vcpu_read/write_page
> > from my series
>
> Hold up on that.  I'd prefer to keep them as there's still value in giving userspace
> debug information.  All I'm proposing is that we would firmly state in the
> documentation that those paths must be treated as informational-only.

Userspace would then need to know whether annotations were performed
from reliable/unreliable paths though, right? That'd imply another
flag bit beyond the current R/W/E bits.

> > - The helper function [a] for filling the memory_fault field
> > (downgraded back into the current union) can drop the "has the field
> > already been filled?" check/WARN.
>
> That would need to be dropped regardless because it's user-triggered (sadly).

Well the current v5 of the series uses a non-userspace visible canary-
it seems like there'd still be value in that if we were to keep the
annotations in potentially unreliable spots. Although perhaps that
test failure you noticed [1] is a good counter-argument, since it
shows a known case where a current flow does multiple writes to the
memory_fault member.

[1] https://lore.kernel.org/all/202309141107.30863e9d-oliver.sang@xxxxxxxxx

> Anyways, don't do anything just yet.

:salutes: