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

[Steven Price]

On 24/06/2020 12:09, Catalin Marinas wrote:
> On Wed, Jun 24, 2020 at 12:03:35PM +0100, Steven Price wrote:
> > On 24/06/2020 11:34, Dave Martin wrote:
> > > On Wed, Jun 24, 2020 at 10:38:48AM +0100, Catalin Marinas wrote:
> > > > On Tue, Jun 23, 2020 at 07:05:07PM +0100, Peter Maydell wrote:
> > > > > On Wed, 17 Jun 2020 at 13:39, Steven Price <steven.price@xxxxxxx> 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.
> > > > >
> > > > > What's your plan for handling tags across VM migration?
> > > > > Will the kernel expose the tag ram to userspace so we
> > > > > can copy it from the source machine to the destination
> > > > > at the same time as we copy the actual ram contents ?
> > > >
> > > > Qemu can map the guest memory with PROT_MTE and access the tags directly
> > > > with LDG/STG instructions. Steven was actually asking in the cover
> > > > letter whether we should require that the VMM maps the guest memory with
> > > > PROT_MTE as a guarantee that it can access the guest tags.
> > > >
> > > > There is no architecturally visible tag ram (tag storage), that's a
> > > > microarchitecture detail.
> > >
> > > If userspace maps the guest memory with PROT_MTE for dump purposes,
> > > isn't it going to get tag check faults when accessing the memory
> > > (i.e., when dumping the regular memory content, not the tags
> > > specifically).
> > >
> > > Does it need to map two aliases, one with PROT_MTE and one without,
> > > and is that architecturally valid?
> >
> > Userspace would either need to have two mappings (I don't believe there are
> > any architectural issues with that - but this could be awkward to arrange in
> > some situations) or be careful to avoid faults. Basically your choices with
> > one mapping are:
> >
> >   1. Disable tag checking (using prctl) when touching the memory. This works
> > but means you lose tag checking for the VMM's own accesses during this code
> > sequence.
> >
> >   2. Read the tag values and ensure you use the correct tag. This suffers
> > from race conditions if the VM is still running.
> >
> >   3. Use one of the exceptions in the architecture that generates a Tag
> > Unchecked access. Sadly the only remotely useful thing I can see in the v8
> > ARM is "A base register plus immediate offset addressing form, with the SP
> > as the base register." - but making sure SP is in range of where you want to
> > access would be a pain.
>
> Or:
>
> 4. Set PSTATE.TCO when accessing tagged memory in an unsafe way.

Ah yes, similar to (1) but much lower overhead ;) That's probably the 
best option - it can be hidden in a memcpy_ignoring_tags() function. 
However it still means that the VMM can't directly touch the guest's 
memory which might cause issues for the VMM.

Steve