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

[Dr. David Alan Gilbert]

* Peter Maydell (peter.maydell@linaro.org) wrote:
> On Mon, 7 Dec 2020 at 16:44, Dr. David Alan Gilbert <dgilbert@redhat.com> 
> wrote:
> > * Steven Price (steven.price@arm.com) wrote:
> > > Sorry, I know I simplified it rather by saying it's similar to protected 
> > > VM.
> > > Basically as I see it there are three types of memory access:
> > >
> > > 1) Debug case - has to go via a special case for decryption or ignoring 
> > > the
> > > MTE tag value. Hopefully this can be abstracted in the same way.
> > >
> > > 2) Migration - for a protected VM there's likely to be a special method to
> > > allow the VMM access to the encrypted memory (AFAIK memory is usually kept
> > > inaccessible to the VMM). For MTE this again has to be special cased as we
> > > actually want both the data and the tag values.
> > >
> > > 3) Device DMA - for a protected VM it's usual to unencrypt a small area of
> > > memory (with the permission of the guest) and use that as a bounce buffer.
> > > This is possible with MTE: have an area the VMM purposefully maps with
> > > PROT_MTE. The issue is that this has a performance overhead and we can do
> > > better with MTE because it's trivial for the VMM to disable the protection
> > > for any memory.
> >
> > Those all sound very similar to the AMD SEV world;  there's the special
> > case for Debug that Peter mentioned; migration is ...complicated and
> > needs special case that's still being figured out, and as I understand
> > Device DMA also uses a bounce buffer (and swiotlb in the guest to make
> > that happen).
> 
> Mmm, but for encrypted VMs the VM has to jump through all these
> hoops because "don't let the VM directly access arbitrary guest RAM"
> is the whole point of the feature. For MTE, we don't want in general
> to be doing tag-checked accesses to guest RAM and there is nothing
> in the feature "allow guests to use MTE" that requires that the VMM's
> guest RAM accesses must do tag-checking. So we should avoid having
> a design that require us to jump through all the hoops.

Yes agreed, that's a fair distinction.

Dave


 Even if
> it happens that handling encrypted VMs means that QEMU has to grow
> some infrastructure for carefully positioning hoops in appropriate
> places, we shouldn't use it unnecessarily... All we actually need is
> a mechanism for migrating the tags: I don't think there's ever a
> situation where you want tag-checking enabled for the VMM's accesses
> to the guest RAM.
> 
> thanks
> -- PMM
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK