On Mon, Dec 10, 2018 at 08:22:06AM -0600, Richard Henderson wrote: > On 12/10/18 6:03 AM, Catalin Marinas wrote: > >> However, it won't be too long before someone implements support for > >> ARMv8.2-LVA, at which point, without changes to mandatory pointer tagging, we > >> will only have 3 authentication bits: [54:52]. This seems useless and easily > >> brute-force-able. > > > > Such support is already here (about to be queued): > > > > https://lore.kernel.org/linux-arm-kernel/20181206225042.11548-1-steve.capper@xxxxxxx/ > > Thanks for the pointer. > > >> Unfortunately, there is no obvious path to making this optional that does not > >> break compatibility with Documentation/arm64/tagged-pointers.txt. > > > > There is also the ARMv8.5 MTE (memory tagging) which relies on tagged > > pointers. > > So it does. I hadn't read through that extension completely before. > > > An alternative would be to allow the opt-in to 52-bit VA, leaving it at > > 48-bit by default. However, it has the problem of changing the PAC size > > and not being able to return. > > Perhaps the opt-in should be at exec time, with ELF flags (or equivalent) on > the application. Because, as you say, changing the shape of the PAC in the > middle of execution is in general not possible. I think we'd still have a potential performance problem with that approach, since we'd end up having to context-switch TCR.T0SZ, which is permitted to be cached in a TLB and would therefore force us to introduce TLB invalidation when context-switching between tasks using 52-bit VAs and tasks using 48-bit VAs. There's a chance we could get the architecture tightened here, but it's not something we've pushed for so far and it depends on what's already been built. Will _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
