On Thu, Oct 10, 2013 at 09:39:55AM +0100, Marc Zyngier wrote: > On 10/10/13 05:51, Anup Patel wrote: > > Are you planning to go ahead with this approach ? > > [adding Catalin, as we heavily discussed this recently] > > Not as such, as it doesn't solve the full issue. It merely papers over > the whole "my cache is off" problem. More specifically, any kind of > speculative access from another CPU while caches are off in the guest > completely nukes the benefit of this patch. > > Also, turning the the caches off is another source of problems, as > speculation also screws up set/way invalidation. Indeed. The set/way operations trapping and broadcasting (or deferring) to other CPUs in software just happens to work but there is no guarantee, sooner or later we'll hit a problem. I'm even tempted to remove flush_dcache_all() calls on the booting path for the arm64 kernel, we already require that whatever runs before Linux should clean&invalidate the caches. Basically, with KVM a VCPU even if running with caches/MMU disabled can still get speculative allocation into the cache. The reason for this is the other cacheable memory aliases created by the host kernel and qemu/kvmtool. I can't tell whether Xen has this issue but it may be easier in Xen to avoid memory aliases. > > We really need this patch for X-Gene L3 cache. > > So far, I can see two possibilities: > - either we mandate caches to be always on (DC bit, and you're not > allowed to turn the caches off). That's my preferred approach. For hotplug, idle, the guest would use an HVC call (PSCI) and the host takes care of re-enabling the DC bit. But we may not catch all cases (kexec probably). > - Or we mandate that caches are invalidated (by VA) for each write that > is performed with caches off. For some things like run-time code patching, on ARMv8 we need to do at least I-cache maintenance since the CPU can allocate into the I-cache (even if there are no aliases). -- Catalin _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/cucslists/listinfo/kvmarm
