linux-kernel - Re: [RFC PATCH V3 0/5] Hi:

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Date:   Mon, 7 Jan 2019 09:47:21 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        davem@...emloft.net
Subject: Re: [RFC PATCH V3 0/5] Hi:

On Mon, Jan 07, 2019 at 02:58:08PM +0800, Jason Wang wrote:
> 
> On 2019/1/5 上午5:41, Michael S. Tsirkin wrote:
> > On Sat, Dec 29, 2018 at 08:46:51PM +0800, Jason Wang wrote:
> > > This series tries to access virtqueue metadata through kernel virtual
> > > address instead of copy_user() friends since they had too much
> > > overheads like checks, spec barriers or even hardware feature
> > > toggling.
> > 
> > I think it's a reasonable approach.
> > However I need to look at whether and which mmu notifiers are invoked before
> > writeback. Do you know?
> 
> 
> I don't know but just looking at the MMU notifier ops definition, there's no
> such callback if my understanding is correct.
> 
> Thanks

In that case how are you making sure used ring updates are written back?
If they aren't guest will crash ...

> 
> > 
> > > Test shows about 24% improvement on TX PPS. It should benefit other
> > > cases as well.
> > > 
> > > Changes from V2:
> > > - fix buggy range overlapping check
> > > - tear down MMU notifier during vhost ioctl to make sure invalidation
> > >    request can read metadata userspace address and vq size without
> > >    holding vq mutex.
> > > Changes from V1:
> > > - instead of pinning pages, use MMU notifier to invalidate vmaps and
> > >    remap duing metadata prefetch
> > > - fix build warning on MIPS
> > > 
> > > Jason Wang (5):
> > >    vhost: generalize adding used elem
> > >    vhost: fine grain userspace memory accessors
> > >    vhost: rename vq_iotlb_prefetch() to vq_meta_prefetch()
> > >    vhost: introduce helpers to get the size of metadata area
> > >    vhost: access vq metadata through kernel virtual address
> > > 
> > >   drivers/vhost/net.c   |   4 +-
> > >   drivers/vhost/vhost.c | 416 +++++++++++++++++++++++++++++++++++++-----
> > >   drivers/vhost/vhost.h |  15 +-
> > >   3 files changed, 384 insertions(+), 51 deletions(-)
> > > 
> > > -- 
> > > 2.17.1