Re: [PATCH 3/5] KVM: PPC: Book3S HV: Improve handling of local vs. global TLB invalidations

Paul Mackerras
2012-11-26T23:16:20Z

On Mon, Nov 26, 2012 at 11:03:19PM +0100, Alexander Graf wrote:
>
> On 26.11.2012, at 22:48, Paul Mackerras wrote:
>
> > On Mon, Nov 26, 2012 at 02:10:33PM +0100, Alexander Graf wrote:
> >>
> >> On 23.11.2012, at 23:07, Paul Mackerras wrote:
> >>
> >>> On Fri, Nov 23, 2012 at 04:43:03PM +0100, Alexander Graf wrote:
> >>>>
> >>>> On 22.11.2012, at 10:28, Paul Mackerras wrote:
> >>>>
> >>>>> - With the possibility of the host paging out guest pages, the use of
> >>>>> H_LOCAL by an SMP guest is dangerous since the guest could possibly
> >>>>> retain and use a stale TLB entry pointing to a page that had been
> >>>>> removed from the guest.
> >>>>
> >>>> I don't understand this part. Don't we flush the TLB when the page gets evicted from the shadow HTAB?
> >>>
> >>> The H_LOCAL flag is something that we invented to allow the guest to
> >>> tell the host "I only ever used this translation (HPTE) on the current
> >>> vcpu" when it's removing or modifying an HPTE. The idea is that that
> >>> would then let the host use the tlbiel instruction (local TLB
> >>> invalidate) rather than the usual global tlbie instruction. Tlbiel is
> >>> faster because it doesn't need to go out on the fabric and get
> >>> processed by all cpus. In fact our guests don't use it at present,
> >>> but we put it in because we thought we should be able to get a
> >>> performance improvement, particularly on large machines.
> >>>
> >>> However, the catch is that the guest's setting of H_LOCAL might be
> >>> incorrect, in which case we could have a stale TLB entry on another
> >>> physical cpu. While the physical page that it refers to is still
> >>> owned by the guest, that stale entry doesn't matter from the host's
> >>> point of view. But if the host wants to take that page away from the
> >>> guest, the stale entry becomes a problem.
> >>
> >> That's exactly where my question lies. Does that mean we don't flush the TLB entry regardless when we take the page away from the guest?
> >
> > The question is how to find the TLB entry if the HPTE it came from is
> > no longer present. Flushing a TLB entry requires a virtual address.
> > When we're taking a page away from the guest we have the real address
> > of the page, not the virtual address. We can use the reverse-mapping
> > chains to loop through all the HPTEs that map the page, and from each
> > HPTE we can (and do) calculate a virtual address and do a TLBIE on
> > that virtual address (each HPTE could be at a different virtual
> > address).
> >
> > The difficulty comes when we no longer have the HPTE but we
> > potentially have a stale TLB entry, due to having used tlbiel when we
> > removed the HPTE. Without the HPTE the only way to get rid of the
> > stale TLB entry would be to completely flush all the TLB entries for
> > the guest's LPID on every physical CPU it had ever run on. Since I
> > don't want to go to that much effort, what I am proposing, and what
> > this patch implements, is to not ever use tlbiel when removing HPTEs
> > in SMP guests on POWER7.
> >
> > In other words, what this patch is about is making sure we don't get
> > these troublesome stale TLB entries.
>
> I see. You could keep a list of to-be-flushed VAs around that you could skim through when taking a page away from the guest. That way you make the fast case fast (add/remove of page from the guest) and the slow path slow (paging).

Yes, I thought about that, but the problem is that the list of VAs
could get arbitrarily long and take up a lot of host memory.

> But I'm fine with disallowing local flushes on remove completely for now. It would be nice to get performance data on how much this would be a net win though. There are certainly ways of keeping local flushes alive with the scheme above.

Yes, I definitely want to get some good performance data to see how
much of a win it would be, and if there is a good win, work out some
scheme to let us use the local flushes.

> Thanks, applied to kvm-ppc-next.

Thanks,
Paul.