Re: [PATCH v15 0/7] MTE support for KVM guest

[Marc Zyngier <maz@kernel.org>]

On Thu, 17 Jun 2021 13:13:22 +0100,
Catalin Marinas <catalin.marinas@arm.com> wrote:
> 
> On Mon, Jun 14, 2021 at 10:05:18AM +0100, Steven Price wrote:
> > I realise there are still open questions[1] around the performance of
> > this series (the 'big lock', tag_sync_lock, introduced in the first
> > patch). But there should be no impact on non-MTE workloads and until we
> > get real MTE-enabled hardware it's hard to know whether there is a need
> > for something more sophisticated or not. Peter Collingbourne's patch[3]
> > to clear the tags at page allocation time should hide more of the impact
> > for non-VM cases. So the remaining concern is around VM startup which
> > could be effectively serialised through the lock.
> [...]
> > [1]: https://lore.kernel.org/r/874ke7z3ng.wl-maz%40kernel.org
> 
> Start-up, VM resume, migration could be affected by this lock, basically
> any time you fault a page into the guest. As you said, for now it should
> be fine as long as the hardware doesn't support MTE or qemu doesn't
> enable MTE in guests. But the problem won't go away.

Indeed. And I find it odd to say "it's not a problem, we don't have
any HW available". By this token, why should we merge this work the
first place, or any of the MTE work that has gone into the kernel over
the past years?

> We have a partial solution with an array of locks to mitigate against
> this but there's still the question of whether we should actually bother
> for something that's unlikely to happen in practice: MAP_SHARED memory
> in guests (ignoring the stage 1 case for now).
> 
> If MAP_SHARED in guests is not a realistic use-case, we have the vma in
> user_mem_abort() and if the VM_SHARED flag is set together with MTE
> enabled for guests, we can reject the mapping.

That's a reasonable approach. I wonder whether we could do that right
at the point where the memslot is associated with the VM, like this:

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index a36a2e3082d8..ebd3b3224386 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -1376,6 +1376,9 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
 		if (!vma)
 			break;
 
+		if (kvm_has_mte(kvm) && vma->vm_flags & VM_SHARED)
+			return -EINVAL;
+
 		/*
 		 * Take the intersection of this VMA with the memory region
 		 */

which takes the problem out of the fault path altogether? We document
the restriction and move on. With that, we can use a non-locking
version of mte_sync_page_tags().

> We can discuss the stage 1 case separately from this series.

Works for me.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.