On Thu, Mar 26, 2015 at 01:58:25PM -0700, Andy Lutomirski wrote: > On Thu, Mar 26, 2015 at 1:31 PM, Radim Krcmar <rkrcmar@xxxxxxxxxx> wrote: > > 2015-03-26 11:51-0700, Andy Lutomirski: > >> On Thu, Mar 26, 2015 at 4:29 AM, Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote: > >> > On Wed, Mar 25, 2015 at 04:22:03PM -0700, Andy Lutomirski wrote: > >> >> Suppose we start out with all vcpus agreeing on their pvti and perfect > >> >> invariant TSCs. Now the host updates its frequency (due to NTP or > >> >> whatever). KVM updates vcpu 0's pvti. Before KVM updates vcpu 1's > >> >> pvti, guest code on vcpus 0 and 1 see synced TSCs but different pvti. > >> >> They'll disagree on the time, and one of them will be ahead until vcpu > >> >> 1's pvti gets updated. > >> > > >> > The masterclock scheme enforces the same system_timestamp/tsc_timestamp pairs > >> > to be visible at one time, for all vcpus. > >> > > >> > > >> > * That is, when timespec0 != timespec1, M < N. Unfortunately that is > >> > * not > >> > * always the case (the difference between two distinct xtime instances > >> > * might be smaller then the difference between corresponding TSC reads, > >> > * when updating guest vcpus pvclock areas). > >> > * > >> > * To avoid that problem, do not allow visibility of distinct > >> > * system_timestamp/tsc_timestamp values simultaneously: use a master > >> > * copy of host monotonic time values. Update that master copy > >> > * in lockstep. > >> > >> Yuck. So we have per cpu timing data, but the protocol is only usable > >> for monotonic timing because we forcibly freeze all vcpus when we > >> update the nominally per cpu data. > >> > >> The obvious guest implementations are still unnecessarily slow, > >> though. It would be nice if the guest could get away without using > >> any getcpu operation at all. > >> > >> Even if we fixed the host to increment version as advertised, I think > >> we can't avoid two getcpu ops. We need one before rdtsc to figure out > >> which pvti to look at, > > > > Yes. > > > >> and we need another to make sure that we were > >> actually on that cpu at the time we did rdtsc. (Rdtscp doesn't help > >> -- we need to check version before rdtsc, and we don't know what > >> version to check until we do a getcpu.). > > > > Exactly, reading cpuid after rdtsc doesn't do that though, we could have > > migrated back between those reads. > > rtdscp would allow us to check that we read tsc of pvti's cpu. > > (It doesn't get rid of that first read.) > > > >> The migration hook has the > >> same issue -- we need to check the migration count, then confirm we're > >> on that cpu, then check the migration count again, and we can't do > >> that until we know what cpu we're on. > > > > True; the revert has a bug -- we need to check cpuid for the second > > time before rdtsc. (Migration hook is there just because we don't know > > which cpu executed rdtsc.) > > One way or another, I'm planning on completely rewriting the vdso > code. An early draft is here: > > https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/commit/?h=x86/vdso&id=57ace6e6e032afc4faf7b9ec52f78a8e6642c980 > > but I can't finish it until the KVM side shakes out. > > I think there are at least two ways that would work: > > a) If KVM incremented version as advertised: All for it. > cpu = getcpu(); > pvti = pvti for cpu; > > ver1 = pvti->version; > check stable bit; > rdtsc_barrier, rdtsc, read scale, shift, etc. > if (getcpu() != cpu) retry; > if (pvti->version != ver1) retry; > > I think this is safe because, we're guaranteed that there was an > interval (between the two version reads) in which the vcpu we think > we're on was running and the kvmclock data was valid and marked > stable, and we know that the tsc we read came from that interval. > > Note: rdtscp isn't needed. If we're stable, is makes no difference > which cpu's tsc we actually read. Yes, can't see a problem with that. > b) If version remains buggy but we use this migrations_from hack: There is no reason for version to remain buggy. > cpu = getcpu(); > pvti = pvti for cpu; > m1 = pvti->migrations_from; > barrier(); > > ver1 = pvti->version; > check stable bit; > rdtsc_barrier, rdtsc, read scale, shift, etc. > if (getcpu() != cpu) retry; > if (pvti->version != ver1) retry; /* probably not really needed */ > > barrier(); > if (pvti->migrations_from != m1) retry; > > This is just like (a), except that we're using a guest kernel hack to > ensure that no one migrated off the vcpu during the version-protected > critical section and that we were, in fact, on that vcpu at some point > during that critical section. Once we've ensured that we were on > pvti's associated vcpu for the entire time we were reading it, then we > are protected by the existing versioning in the host. > > > > >> If, on the other hand, we could rely on having all of these things in > >> sync, then this complication goes away, and we go down from two getcpu > >> ops to zero. > > > > (Yeah, we should look what are the drawbacks of doing it differently.) > > If the versioning were fixed, I think we could almost get away with: > > pvti = pvti for vcpu 0; > > ver1 = pvti->version; > check stable bit; > rdtsc_barrier, rdtsc, read scale, shift, etc. > if (pvti->version != ver1) retry; > > This guarantees that the tsc came from an interval in which vcpu0's > kvmclock was *marked* stable. If vcpu0's kvmclock were genuinely > stable in that interval, then we'd be fine, but there's a race window > in which the kvmclock is *not* stable and vcpu 0 wasn't running. What is that window again ? Have no objections against using vcpu0's pvti (cacheline should be read-only 99.9% of time). > Why doesn't KVM just update all of the kvmclock data at once? Because it has not been necessary -- updating kvmclock data on vcpu entry was the previous method, so that was reused. > (For > that matter, why is the pvti in guest memory at all? Wouldn't this > all be simpler if the kvmclock data were host-allocated so the host > could write it directly and maybe even share it between guests?) And use a 4K TLB entry for that kvmclock area rather than sharing one of kernel's 2MB (or 1GB) TLB entry? > > --Andy -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
