Re: [RFC PATCH V3 0/5] Hi:

Michael S. Tsirkin
2019-01-07T14:47:21Z

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