Re: [PATCH v5 0/2] MTE support for KVM guest

[Catalin Marinas <catalin.marinas@arm.com>]

On Mon, Dec 07, 2020 at 07:03:13PM +0000, Marc Zyngier wrote:
> On Mon, 07 Dec 2020 16:34:05 +0000,
> Catalin Marinas <catalin.marinas@arm.com> wrote:
> > On Mon, Dec 07, 2020 at 04:05:55PM +0000, Marc Zyngier wrote:
> > > What I'd really like to see is a description of how shared memory
> > > is, in general, supposed to work with MTE. My gut feeling is that
> > > it doesn't, and that you need to turn MTE off when sharing memory
> > > (either implicitly or explicitly).
> > 
> > The allocation tag (in-memory tag) is a property assigned to a physical
> > address range and it can be safely shared between different processes as
> > long as they access it via pointers with the same allocation tag (bits
> > 59:56). The kernel enables such tagged shared memory for user processes
> > (anonymous, tmpfs, shmem).
> 
> I think that's one case where the shared memory scheme breaks, as we
> have two kernels in charge of their own tags, and they obviously can't
> be synchronised

Yes, if you can't trust the other entity to not change the tags, the
only option is to do an untagged access.

> > What we don't have in the architecture is a memory type which allows
> > access to tags but no tag checking. To access the data when the tags
> > aren't known, the tag checking would have to be disabled via either a
> > prctl() or by setting the PSTATE.TCO bit.
> 
> I guess that's point (3) in Steven's taxonomy. It still a bit ugly to
> fit in an existing piece of userspace, specially if it wants to use
> MTE for its own benefit.

I agree it's ugly. For the device DMA emulation case, the only sane way
is to mimic what a real device does - no tag checking. For a generic
implementation, this means that such shared memory should not be mapped
with PROT_MTE on the VMM side. I guess this leads to your point that
sharing doesn't work for this scenario ;).

> > The kernel accesses the user memory via the linear map using a match-all
> > tag 0xf, so no TCO bit toggling. For user, however, we disabled such
> > match-all tag and it cannot be enabled at run-time (at least not easily,
> > it's cached in the TLB). However, we already have two modes to disable
> > tag checking which Qemu could use when migrating data+tags.
> 
> I wonder whether we will have to have something kernel side to
> dump/reload tags in a way that matches the patterns used by live
> migration.

We have something related - ptrace dumps/resores the tags. Can the same
concept be expanded to a KVM ioctl?

-- 
Catalin