* 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. 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 _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
