Re: [RFC PATCH 0/2] MTE support for KVM guest

From: Steven Price
Date: Wed Jun 24 2020 - 07:16:41 EST

On 23/06/2020 18:48, Catalin Marinas wrote:
Hi Steven,

On Wed, Jun 17, 2020 at 01:38:42PM +0100, Steven Price wrote:
These patches add support to KVM to enable MTE within a guest. It is
based on Catalin's v4 MTE user space series[1].

[1] http://lkml.kernel.org/r/20200515171612.1020-1-catalin.marinas%40arm.com

Posting as an RFC as I'd like feedback on the approach taken. First a
little background on how MTE fits within the architecture:

The stage 2 page tables have limited scope for controlling the
availability of MTE. If a page is mapped as Normal and cached in stage 2
then it's the stage 1 tables that get to choose whether the memory is
tagged or not. So the only way of forbidding tags on a page from the
hypervisor is to change the cacheability (or make it device memory)
which would cause other problems. Note this restriction fits the
intention that a system should have all (general purpose) memory
supporting tags if it support MTE, so it's not too surprising.

However, the upshot of this is that to enable MTE within a guest all
pages of memory mapped into the guest as normal cached pages in stage 2
*must* support MTE (i.e. we must ensure the tags are appropriately
sanitised and save/restore the tags during swap etc).

My current approach is that KVM transparently upgrades any pages
provided by the VMM to be tag-enabled when they are faulted in (i.e.
sets the PG_mte_tagged flag on the page) which has the benefit of
requiring fewer changes in the VMM. However, save/restore of the VM
state still requires the VMM to have a PROT_MTE enabled mapping so that
it can access the tag values. A VMM which 'forgets' to enable PROT_MTE
would lose the tag values when saving/restoring (tags are RAZ/WI when
PROT_MTE isn't set).

An alternative approach would be to enforce the VMM provides PROT_MTE
memory in the first place. This seems appealing to prevent the above
potentially unexpected gotchas with save/restore, however this would
also extend to memory that you might not expect to have PROT_MTE (e.g. a
shared frame buffer for an emulated graphics card).

As you mentioned above, if memory is mapped as Normal Cacheable at Stage
2 (whether we use FWB or not), the guest is allowed to turn MTE on via
Stage 1. There is no way for KVM to prevent a guest from using MTE other
than the big HCR_EL2.ATA knob.

This causes potential issues since we can't guarantee that all the
Cacheable memory slots allocated by the VMM support MTE. If they do not,
the arch behaviour is "unpredictable". We also can't trust the guest to
not enable MTE on such Cacheable mappings.

Architecturally it seems dodgy to export any address that isn't "normal memory" (i.e. with tag storage) to the guest as Normal Cacheable. Although I'm a bit worried this might cause a regression in some existing case.

On the host kernel, mmap'ing with PROT_MTE is only allowed for anonymous
mappings and shmem. So requiring the VMM to always pass PROT_MTE mapped
ranges to KVM, irrespective of whether it's guest RAM, emulated device,
virtio etc. (as long as they are Cacheable), filters unsafe ranges that
may be mapped into guest.

That would be an easy way of doing the filtering, but it's not clear whether PROT_MTE is actually what the VMM wants (it most likely doesn't want to have tag checking enabled on the memory in user space).

Note that in the next revision of the MTE patches I'll drop the DT
memory nodes checking and rely only on the CPUID information (arch
updated promised by the architects).

I see two possible ways to handle this (there may be more):

1. As in your current patches, assume any Cacheable at Stage 2 can have
MTE enabled at Stage 1. In addition, we need to check whether the
physical memory supports MTE and it could be something simple like
pfn_valid(). Is there a way to reject a memory slot passed by the
VMM?

Yes pfn_valid() should have been in there. At the moment pfn_to_page() is called without any checks.

The problem with attempting to reject a memory slot is that the memory backing that slot can change. So checking at the time the slot is created isn't enough (although it might be a useful error checking feature).

It's not clear to me what we can do at fault time when we discover the memory isn't tag-capable and would have been mapped cacheable other than kill the VM.

2. Similar to 1 but instead of checking whether the pfn supports MTE, we
require the VMM to only pass PROT_MTE ranges (filtering already done
by the host kernel). We need a way to reject the slot and return an
error to the VMM.

I think rejecting a slot at the Stage 2 fault time is very late. You
probably won't be able to do much other than killing the guest.

As above, we will struggle to catch all cases during slot creation, so I think we're going to have to deal with this late detection as well.

Both 1 and 2 above risk breaking existing VMMs just because they happen
to start on an MTE-capable machine. So, can we also require the VMM to
explicitly opt in to MTE support in guests via some ioctl()? This in
turn would enable the additional checks in KVM for the MTE capability of
the memory slots (1 or 2 above).

Patch 2 introduces a VCPU feature which must be explicitly enabled for the guest to have MTE. So it won't break existing VMMs. However clearly simply setting that bit will likely break some configurations where not all memory is MTE capable.

An alternative to an MTE enable ioctl(), if all the memory slots are set
up prior to the VM starting, KVM could check 1 or 2 above and decide
whether to expose MTE to guests (HCR_EL2.ATA).

The VMM needs to be fully aware of MTE before it's enabled by KVM otherwise it could lose the tags (e.g. during migration). So my preference is to make it an explicit opt-in.

More questions than solutions above, mostly for the KVM and Qemu
maintainers.

Indeed it would be useful to have input from those familiar with VMMs as to what a good interface looks like.

Thanks,

Steve