Re: [PATCH 1/3] KVM: arm64: Fix S1PTW handling on RO memslots

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

On Wed, Dec 21, 2022 at 05:53:58PM +0000, Marc Zyngier wrote:
> On Wed, 21 Dec 2022 16:50:30 +0000,
> Oliver Upton <oliver.upton@linux.dev> wrote:
> > 
> > On Wed, Dec 21, 2022 at 09:35:06AM +0000, Marc Zyngier wrote:
> > 
> > [...]
> > 
> > > > > +	if (kvm_vcpu_abt_iss1tw(vcpu)) {
> > > > > +		/*
> > > > > +		 * Only a permission fault on a S1PTW should be
> > > > > +		 * considered as a write. Otherwise, page tables baked
> > > > > +		 * in a read-only memslot will result in an exception
> > > > > +		 * being delivered in the guest.
> > > > 
> > > > Somewhat of a tangent, but:
> > > > 
> > > > Aren't we somewhat unaligned with the KVM UAPI by injecting an
> > > > exception in this case? I know we've been doing it for a while, but it
> > > > flies in the face of the rules outlined in the
> > > > KVM_SET_USER_MEMORY_REGION documentation.
> > > 
> > > That's an interesting point, and I certainly haven't considered that
> > > for faults introduced by page table walks.
> > > 
> > > I'm not sure what userspace can do with that though. The problem is
> > > that this is a write for which we don't have useful data: although we
> > > know it is a page-table walker access, we don't know what it was about
> > > to write. The instruction that caused the write is meaningless (it
> > > could either be a load, a store, or an instruction fetch). How do you
> > > populate the data[] field then?
> > > 
> > > If anything, this is closer to KVM_EXIT_ARM_NISV, for which we give
> > > userspace the full ESR and ask it to sort it out. I doubt it will be
> > > able to, but hey, maybe it is worth a shot. This would need to be a
> > > different exit reason though, as NISV is explicitly for non-memslot
> > > stuff.
> > > 
> > > In any case, the documentation for KVM_SET_USER_MEMORY_REGION needs to
> > > reflect the fact that KVM_EXIT_MMIO cannot represent a fault due to a
> > > S1 PTW.
> > 
> > Oh I completely agree with you here. I probably should have said before,
> > I think the exit would be useless anyway. Getting the documentation in
> > line with the intended behavior seems to be the best fix.
> 
> Right. How about something like this?

Looks good to me, thanks!

Reviewed-by: Oliver Upton <oliver.upton@linux.dev>

> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 226b40baffb8..72abd018a618 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -1381,6 +1381,14 @@ It is recommended to use this API instead of the KVM_SET_MEMORY_REGION ioctl.
>  The KVM_SET_MEMORY_REGION does not allow fine grained control over memory
>  allocation and is deprecated.
>  
> +Note: On arm64, a write generated by the page-table walker (to update
> +the Access and Dirty flags, for example) never results in a
> +KVM_EXIT_MMIO exit when the slot has the KVM_MEM_READONLY flag. This
> +is because KVM cannot provide the data that would be written by the
> +page-table walker, making it impossible to emulate the access.
> +Instead, an abort (data abort if the cause of the page-table update
> +was a load or a store, instruction abort if it was an instruction
> +fetch) is injected in the guest.
>  
>  4.36 KVM_SET_TSS_ADDR
>  ---------------------

--
Best,
Oliver

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