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Date: Wed, 28 Dec 2022 02:20:19 -0500
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Shunsuke Mie <mie@...l.co.jp>
Cc: Jason Wang <jasowang@...hat.com>,
Rusty Russell <rusty@...tcorp.com.au>, kvm@...r.kernel.org,
virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 4/9] vringh: unify the APIs for all accessors
On Wed, Dec 28, 2022 at 11:24:10AM +0900, Shunsuke Mie wrote:
> 2022年12月27日(火) 23:37 Michael S. Tsirkin <mst@...hat.com>:
> >
> > On Tue, Dec 27, 2022 at 07:22:36PM +0900, Shunsuke Mie wrote:
> > > 2022年12月27日(火) 16:49 Shunsuke Mie <mie@...l.co.jp>:
> > > >
> > > > 2022年12月27日(火) 16:04 Michael S. Tsirkin <mst@...hat.com>:
> > > > >
> > > > > On Tue, Dec 27, 2022 at 11:25:26AM +0900, Shunsuke Mie wrote:
> > > > > > Each vringh memory accessors that are for user, kern and iotlb has own
> > > > > > interfaces that calls common code. But some codes are duplicated and that
> > > > > > becomes loss extendability.
> > > > > >
> > > > > > Introduce a struct vringh_ops and provide a common APIs for all accessors.
> > > > > > It can bee easily extended vringh code for new memory accessor and
> > > > > > simplified a caller code.
> > > > > >
> > > > > > Signed-off-by: Shunsuke Mie <mie@...l.co.jp>
> > > > > > ---
> > > > > > drivers/vhost/vringh.c | 667 +++++++++++------------------------------
> > > > > > include/linux/vringh.h | 100 +++---
> > > > > > 2 files changed, 225 insertions(+), 542 deletions(-)
> > > > > >
> > > > > > diff --git a/drivers/vhost/vringh.c b/drivers/vhost/vringh.c
> > > > > > index aa3cd27d2384..ebfd3644a1a3 100644
> > > > > > --- a/drivers/vhost/vringh.c
> > > > > > +++ b/drivers/vhost/vringh.c
> > > > > > @@ -35,15 +35,12 @@ static __printf(1,2) __cold void vringh_bad(const char *fmt, ...)
> > > > > > }
> > > > > >
> > > > > > /* Returns vring->num if empty, -ve on error. */
> > > > > > -static inline int __vringh_get_head(const struct vringh *vrh,
> > > > > > - int (*getu16)(const struct vringh *vrh,
> > > > > > - u16 *val, const __virtio16 *p),
> > > > > > - u16 *last_avail_idx)
> > > > > > +static inline int __vringh_get_head(const struct vringh *vrh, u16 *last_avail_idx)
> > > > > > {
> > > > > > u16 avail_idx, i, head;
> > > > > > int err;
> > > > > >
> > > > > > - err = getu16(vrh, &avail_idx, &vrh->vring.avail->idx);
> > > > > > + err = vrh->ops.getu16(vrh, &avail_idx, &vrh->vring.avail->idx);
> > > > > > if (err) {
> > > > > > vringh_bad("Failed to access avail idx at %p",
> > > > > > &vrh->vring.avail->idx);
> > > > >
> > > > > I like that this patch removes more lines of code than it adds.
> > > > >
> > > > > However one of the design points of vringh abstractions is that they were
> > > > > carefully written to be very low overhead.
> > > > > This is why we are passing function pointers to inline functions -
> > > > > compiler can optimize that out.
> > > > >
> > > > > I think that introducing ops indirect functions calls here is going to break
> > > > > these assumptions and hurt performance.
> > > > > Unless compiler can somehow figure it out and optimize?
> > > > > I don't see how it's possible with ops pointer in memory
> > > > > but maybe I'm wrong.
> > > > I think your concern is correct. I have to understand the compiler
> > > > optimization and redesign this approach If it is needed.
> > > > > Was any effort taken to test effect of these patches on performance?
> > > > I just tested vringh_test and already faced little performance reduction.
> > > > I have to investigate that, as you said.
> > > I attempted to test with perf. I found that the performance of patched code
> > > is almost the same as the upstream one. However, I have to investigate way
> > > this patch leads to this result, also the profiling should be run on
> > > more powerful
> > > machines too.
> > >
> > > environment:
> > > $ grep 'model name' /proc/cpuinfo
> > > model name : Intel(R) Core(TM) i3-7020U CPU @ 2.30GHz
> > > model name : Intel(R) Core(TM) i3-7020U CPU @ 2.30GHz
> > > model name : Intel(R) Core(TM) i3-7020U CPU @ 2.30GHz
> > > model name : Intel(R) Core(TM) i3-7020U CPU @ 2.30GHz
> > >
> > > results:
> > > * for patched code
> > > Performance counter stats for 'nice -n -20 ./vringh_test_patched
> > > --parallel --eventidx --fast-vringh --indirect --virtio-1' (20 runs):
> > >
> > > 3,028.05 msec task-clock # 0.995 CPUs
> > > utilized ( +- 0.12% )
> > > 78,150 context-switches # 25.691 K/sec
> > > ( +- 0.00% )
> > > 5 cpu-migrations # 1.644 /sec
> > > ( +- 3.33% )
> > > 190 page-faults # 62.461 /sec
> > > ( +- 0.41% )
> > > 6,919,025,222 cycles # 2.275 GHz
> > > ( +- 0.13% )
> > > 8,990,220,160 instructions # 1.29 insn per
> > > cycle ( +- 0.04% )
> > > 1,788,326,786 branches # 587.899 M/sec
> > > ( +- 0.05% )
> > > 4,557,398 branch-misses # 0.25% of all
> > > branches ( +- 0.43% )
> > >
> > > 3.04359 +- 0.00378 seconds time elapsed ( +- 0.12% )
> > >
> > > * for upstream code
> > > Performance counter stats for 'nice -n -20 ./vringh_test_base
> > > --parallel --eventidx --fast-vringh --indirect --virtio-1' (10 runs):
> > >
> > > 3,058.41 msec task-clock # 0.999 CPUs
> > > utilized ( +- 0.14% )
> > > 78,149 context-switches # 25.545 K/sec
> > > ( +- 0.00% )
> > > 5 cpu-migrations # 1.634 /sec
> > > ( +- 2.67% )
> > > 194 page-faults # 63.414 /sec
> > > ( +- 0.43% )
> > > 6,988,713,963 cycles # 2.284 GHz
> > > ( +- 0.14% )
> > > 8,512,533,269 instructions # 1.22 insn per
> > > cycle ( +- 0.04% )
> > > 1,638,375,371 branches # 535.549 M/sec
> > > ( +- 0.05% )
> > > 4,428,866 branch-misses # 0.27% of all
> > > branches ( +- 22.57% )
> > >
> > > 3.06085 +- 0.00420 seconds time elapsed ( +- 0.14% )
> >
> >
> > How you compiled it also matters. ATM we don't enable retpolines
> > and it did not matter since we didn't have indirect calls,
> > but we should. Didn't yet investigate how to do that for virtio tools.
> I think the retpolines certainly affect performance. Thank you for pointing
> it out. I'd like to start the investigation that how to apply the
> retpolines to the
> virtio tools.
> > > > Thank you for your comments.
> > > > > Thanks!
> > > > >
> > > > >
> > > > Best,
> > > > Shunsuke.
This isn't all that trivial if we want this at runtime.
But compile time is kind of easy.
See Documentation/admin-guide/hw-vuln/spectre.rst
--
MST
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