Re: [RFC 2/2] x86, vdso, pvclock: Simplify and speed up the vdso pvclock reader

[Marcelo Tosatti]

On Tue, Jan 06, 2015 at 10:26:22AM -0800, Andy Lutomirski wrote:
> On Tue, Jan 6, 2015 at 10:13 AM, Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote:
> > On Tue, Jan 06, 2015 at 08:56:40AM -0800, Andy Lutomirski wrote:
> >> On Jan 6, 2015 4:01 AM, "Paolo Bonzini" <pbonzini@xxxxxxxxxx> wrote:
> >> >
> >> >
> >> >
> >> > On 06/01/2015 09:42, Paolo Bonzini wrote:
> >> > > > > Still confused.  So we can freeze all vCPUs in the host, then update
> >> > > > > pvti 1, then resume vCPU 1, then update pvti 0?  In that case, we have
> >> > > > > a problem, because vCPU 1 can observe pvti 0 mid-update, and KVM
> >> > > > > doesn't increment the version pre-update, and we can return completely
> >> > > > > bogus results.
> >> > > > Yes.
> >> > > But then the getcpu test would fail (1->0).  Even if you have an ABA
> >> > > situation (1->0->1), it's okay because the pvti that is fetched is the
> >> > > one returned by the first getcpu.
> >> >
> >> > ... this case of partial update of pvti, which is caught by the version
> >> > field, if of course different from the other (extremely unlikely) that
> >> > Andy pointed out.  That is when the getcpus are done on the same vCPU,
> >> > but the rdtsc is another.
> >> >
> >> > That one can be fixed by rdtscp, like
> >> >
> >> > do {
> >> >     // get a consistent (pvti, v, tsc) tuple
> >> >     do {
> >> >         cpu = get_cpu();
> >> >         pvti = get_pvti(cpu);
> >> >         v = pvti->version & ~1;
> >> >         // also acts as rmb();
> >> >         rdtsc_barrier();
> >> >         tsc = rdtscp(&cpu1);
> >>
> >> Off-topic note: rdtscp doesn't need a barrier at all.  AIUI AMD
> >> specified it that way and both AMD and Intel implement it correctly.
> >> (rdtsc, on the other hand, definitely needs the barrier beforehand.)
> >>
> >> >         // control dependency, no need for rdtsc_barrier?
> >> >     } while(cpu != cpu1);
> >> >
> >> >     // ... compute nanoseconds from pvti and tsc ...
> >> >     rmb();
> >> > }   while(v != pvti->version);
> >>
> >> Still no good.  We can migrate a bunch of times so we see the same CPU
> >> all three times and *still* don't get a consistent read, unless we
> >> play nasty games with lots of version checks (I have a patch for that,
> >> but I don't like it very much).  The patch is here:
> >>
> >> https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/commit/?h=x86/vdso_paranoia&id=a69754dc5ff33f5187162b5338854ad23dd7be8d
> >>
> >> but I don't like it.
> >>
> >> Thus far, I've been told unambiguously that a guest can't observe pvti
> >> while it's being written, and I think you're now telling me that this
> >> isn't true and that a guest *can* observe pvti while it's being
> >> written while the low bit of the version field is not set.  If so,
> >> this is rather strongly incompatible with the spec in the KVM docs.
> >>
> >> I don't suppose that you and Marcelo could agree on what the actual
> >> semantics that KVM provides are and could write it down in a way that
> >> people who haven't spent a long time staring at the request code
> >> understand?  And maybe you could even fix the implementation while
> >> you're at it if the implementation is, indeed, broken.  I have ugly
> >> patches to fix it here:
> >>
> >> https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/commit/?h=x86/vdso_paranoia&id=3b718a050cba52563d831febc2e1ca184c02bac0
> >>
> >> but I'm not thrilled with them.
> >>
> >> --Andy
> >
> > I suppose that separating the version write from the rest of the pvclock
> > structure is sufficient, as that would guarantee the writes are not
> > reordered even with fast string REP MOVS.
> >
> > Thanks for catching this Andy!
> >
> 
> Don't you stil need:
> 
> version++;
> write the rest;
> version++;
> 
> with possible smp_wmb() in there to keep the compiler from messing around?

Correct. Could just as well follow the protocol and use odd/even, which 
is what your patch does.

What is the point with the new flags bit though?

> Also, if you do this, can you also make setting and clearing
> STABLE_BIT properly atomic across all vCPUs?  Or at least do something
> like setting it last and clearing it first on vPCU 0?

If the version "seqlock" works properly across vCPUs, why do you need
STABLE_BIT "properly atomic" ?

Please define what you mean by "properly atomic".


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