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Re: [qemu-s390x] [Qemu-arm] [RFC v4 00/71] per-CPU locks

From: Emilio G. Cota
Subject: Re: [qemu-s390x] [Qemu-arm] [RFC v4 00/71] per-CPU locks
Date: Mon, 29 Oct 2018 11:47:28 -0400
User-agent: Mutt/1.9.4 (2018-02-28)

On Sat, Oct 27, 2018 at 10:14:47 +0100, Alex Bennée wrote:
> Emilio G. Cota <address@hidden> writes:
> > [I forgot to add the cover letter to git send-email; here it is]
> >
> > v3: https://lists.gnu.org/archive/html/qemu-devel/2018-10/msg04179.html
> >
> > "Why is this an RFC?" See v3 link above. Also, see comment at
> > the bottom of this message regarding the last patch of this series.
> I'm also seeing his hang on check-tcg, specifically qemu-aarch64 
> ./tests/linux-test

Thanks for reporting. The last patch in the series is the one
that causes the hang. I didn't test that patch much, since I
did not intend to get it merged.

Over the weekend I had a bit of time to think about an actual fix, i.e.
how to reduce safe work calls for TLB invalidations. The idea is to check
whether the remote invalidation is necessary at all; we can take the remote
tlb's lock, and check whether the address we want to invalidate has been
read by the remote CPU since its latest flush. On some quick tests
booting an aarch64 system I measured that only up to ~2% of remote
invalidations are actually necessary.

I just did a search on google scholar and found a similar approach
to reduce remote TLB shootdowns on ARM, this time for hardware.
This paper
  "TLB Shootdown Mitigation for Low-Power Many-Core Servers with 
  L1 Virtual Caches"
addresses the issue by employing bloom filters in hardware to
determine whether an address has been accessed by a TLB before
performing an invalidation (and the corresponding icache flush).

In software, using a per-TLB hash table might be enough. I'll try
to have something ready for v5.



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