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

[Steven Price]

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.

The kernel could provide a mechanism to do this, but I'm not sure that 
would be better than 1.

This, however, is another argument for my current approach of 
"upgrading" the pages automatically and not forcing the VMM to set 
PROT_MTE. But in this case we probably would need a kernel interface to 
fetch the tags as the VM sees them.

Steve