On Mon, Dec 07, 2020 at 07:03:13PM +0000, Marc Zyngier wrote: > On Mon, 07 Dec 2020 16:34:05 +0000, > Catalin Marinas <catalin.marinas@xxxxxxx> wrote: > > On Mon, Dec 07, 2020 at 04:05:55PM +0000, Marc Zyngier wrote: > > > What I'd really like to see is a description of how shared memory > > > is, in general, supposed to work with MTE. My gut feeling is that > > > it doesn't, and that you need to turn MTE off when sharing memory > > > (either implicitly or explicitly). > > > > The allocation tag (in-memory tag) is a property assigned to a physical > > address range and it can be safely shared between different processes as > > long as they access it via pointers with the same allocation tag (bits > > 59:56). The kernel enables such tagged shared memory for user processes > > (anonymous, tmpfs, shmem). > > I think that's one case where the shared memory scheme breaks, as we > have two kernels in charge of their own tags, and they obviously can't > be synchronised Yes, if you can't trust the other entity to not change the tags, the only option is to do an untagged access. > > What we don't have in the architecture is a memory type which allows > > access to tags but no tag checking. To access the data when the tags > > aren't known, the tag checking would have to be disabled via either a > > prctl() or by setting the PSTATE.TCO bit. > > I guess that's point (3) in Steven's taxonomy. It still a bit ugly to > fit in an existing piece of userspace, specially if it wants to use > MTE for its own benefit. I agree it's ugly. For the device DMA emulation case, the only sane way is to mimic what a real device does - no tag checking. For a generic implementation, this means that such shared memory should not be mapped with PROT_MTE on the VMM side. I guess this leads to your point that sharing doesn't work for this scenario ;). > > The kernel accesses the user memory via the linear map using a match-all > > tag 0xf, so no TCO bit toggling. For user, however, we disabled such > > match-all tag and it cannot be enabled at run-time (at least not easily, > > it's cached in the TLB). However, we already have two modes to disable > > tag checking which Qemu could use when migrating data+tags. > > I wonder whether we will have to have something kernel side to > dump/reload tags in a way that matches the patterns used by live > migration. We have something related - ptrace dumps/resores the tags. Can the same concept be expanded to a KVM ioctl? -- Catalin _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
