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 > 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. > 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. M. -- Without deviation from the norm, progress is not possible. _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
