On 9/4/20 9:00 AM, Steven Price wrote: > 3. Doesn't provide any new methods for the VMM to access the tags on > memory. ... > (3) may be problematic and I'd welcome input from those familiar with > VMMs. User space cannot access tags unless the memory is mapped with the > PROT_MTE flag. However enabling PROT_MTE will also enable tag checking > for the user space process (assuming the VMM enables tag checking for > the process)... The latest version of the kernel patches for user mte support has separate controls for how tag check fail is reported. Including > +- ``PR_MTE_TCF_NONE`` - *Ignore* tag check faults That may be less than optimal once userland starts uses tags itself, e.g. running qemu itself with an mte-aware malloc. Independent of that, there's also the TCO bit, which can be toggled by any piece of code that wants to disable checking locally. However, none of that is required for accessing tags. User space can always load/store tags via LDG/STG. That's going to be slow, though. It's a shame that LDGM/STGM are privileged instructions. I don't understand why that was done, since there's absolutely nothing that those insns can do that you can't do with (up to) 16x LDG/STG. I think it might be worth adding some sort of kernel entry point that can bulk copy tags, e.g. page aligned quantities. But that's just a speed of migration thing and could come later. r~ _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
