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.
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
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