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

[Haibo Xu]

On Tue, 8 Dec 2020 at 00:44, Dr. David Alan Gilbert <dgilbert@xxxxxxxxxx> wrote:
>
> * Steven Price (steven.price@xxxxxxx) wrote:
> > On 07/12/2020 15:27, Peter Maydell wrote:
> > > On Mon, 7 Dec 2020 at 14:48, Steven Price <steven.price@xxxxxxx> wrote:
> > > > Sounds like you are making good progress - thanks for the update. Have
> > > > you thought about how the PROT_MTE mappings might work if QEMU itself
> > > > were to use MTE? My worry is that we end up with MTE in a guest
> > > > preventing QEMU from using MTE itself (because of the PROT_MTE
> > > > mappings). I'm hoping QEMU can wrap its use of guest memory in a
> > > > sequence which disables tag checking (something similar will be needed
> > > > for the "protected VM" use case anyway), but this isn't something I've
> > > > looked into.
> > >
> > > It's not entirely the same as the "protected VM" case. For that
> > > the patches currently on list basically special case "this is a
> > > debug access (eg from gdbstub/monitor)" which then either gets
> > > to go via "decrypt guest RAM for debug" or gets failed depending
> > > on whether the VM has a debug-is-ok flag enabled. For an MTE
> > > guest the common case will be guests doing standard DMA operations
> > > to or from guest memory. The ideal API for that from QEMU's
> > > point of view would be "accesses to guest RAM don't do tag
> > > checks, even if tag checks are enabled for accesses QEMU does to
> > > memory it has allocated itself as a normal userspace program".
> >
> > 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).
>
>
> I'm not sure about the stories for the IBM hardware equivalents.

Like s390-skeys(storage keys) support in Qemu?

I have read the migration support for the s390-skeys in Qemu and found
that the logic is very similar to that of MTE, except the difference that the
s390-skeys were migrated separately from that of the guest memory data
while for MTE, I think the guest memory tags should go with the  memory data.

>
> Dave
>
> > The part I'm unsure on is how easy it is for QEMU to deal with (3) without
> > the overhead of bounce buffers. Ideally there'd already be a wrapper for
> > guest memory accesses and that could just be wrapped with setting TCO during
> > the access. I suspect the actual situation is more complex though, and I'm
> > hoping Haibo's investigations will help us understand this.
> >
> > Thanks,
> >
> > Steve
> >
> --
> Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK
>