Re: [PATCH RFC v2 0/2] arm: enable MTE for QEMU + kvm

[Dr. David Alan Gilbert]

* Cornelia Huck (cohuck@redhat.com) wrote:
> On Mon, Jul 11 2022, "Dr. David Alan Gilbert" <dgilbert@redhat.com> wrote:
> 
> > * Peter Maydell (peter.maydell@linaro.org) wrote:
> >> On Mon, 11 Jul 2022 at 14:24, Dr. David Alan Gilbert
> >> <dgilbert@redhat.com> wrote:
> >> > But, ignoring postcopy for a minute, with KVM how do different types of
> >> > backing memory work - e.g. if I back a region of guest memory with
> >> > /dev/shm/something or a hugepage equivalent, where does the MTE memory
> >> > come from, and how do you set it?
> >> 
> >> Generally in an MTE system anything that's "plain old RAM" is expected
> >> to support tags. (The architecture manual calls this "conventional
> >> memory". This isn't quite the same as "anything that looks RAM-like",
> >> e.g. the graphics card framebuffer doesn't have to support tags!)
> >
> > I guess things like non-volatile disks mapped as DAX are fun edge cases.
> >
> >> One plausible implementation is that the firmware and memory controller
> >> are in cahoots and arrange that the appropriate fraction of the DRAM is
> >> reserved for holding tags (and inaccessible as normal RAM even by the OS);
> >> but where the tags are stored is entirely impdef and an implementation
> >> could choose to put the tags in their own entirely separate storage if
> >> it liked. The only way to access the tag storage is via the instructions
> >> for getting and setting tags.
> >
> > Hmm OK;   In postcopy, at the moment, the call qemu uses is a call that
> > atomically places a page of data in memory and then tells the vCPUs to
> > continue.  I guess a variant that took an extra blob of MTE data would
> > do.
> 
> Yes, the current idea is to extend UFFDIO_COPY with a flag so that we
> get the tag data along with the page.
> 
> > Note that other VMMs built on kvm work in different ways; the other
> > common way is to write into the backing file (i.e. the /dev/shm
> > whatever atomically somehow) and then do the userfault call to tell the
> > vcpus to continue.  It looks like this is the way things will work in
> > the split hugepage mechanism Google are currently adding.
> 
> Hmm... I had the impression that other VMMs had not cared about this
> particular use case yet; if they need a slightly different mechanism,
> it would complicate things a bit.

I think Google's internal VMM doesn't use UFFDIO_COPY - but I don't have
details to be sure of that.

Dave

-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK