Re: [PATCH 0/2] exclude hyperv synic sections from vhost

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Jason Wang (jasowang@redhat.com) wrote:
> 
> On 2020/1/8 下午9:53, Dr. David Alan Gilbert (git) wrote:
> > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> > 
> > Hyperv's synic (that we emulate) is a feature that allows the guest
> > to place some magic (4k) pages of RAM anywhere it likes in GPA.
> > This confuses vhost's RAM section merging when these pages
> > land over the top of hugepages.
> 
> 
> Hi David:
> 
> A silly question, is this because the alignment when adding sections? If
> yes, what's the reason for doing alignment which is not a must for vhost
> memory table.

Page alignment is a bit odd with vhost-user - it ends up having to mmap
each of the sections itself; and still has to map them as hugepages
to be able to mmap - in the old world you could sometimes have
the daemon mmaping the same chunk of memory twice into the vhost-user
process; without the aggregation  you'd get a hugepage mapping for the
0-2MB chunk for the 0-512K mapping, and then maybe another 0-2MB chunk
for some of the other parts over 512K.
With postcopy we can't have the multiple mappings of the same part of
guest memory; we need to have one mapping for userfault.

Also, given the 16 separate synic regions, you'd probably end up having
a lot of wasted vhost-sections.

Dave




> Thanks
> 
> 
> > 
> > Since they're not normal RAM, and they shouldn't have vhost DMAing
> > into them, exclude them from the vhost set.
> > 
> > I do that by marking them as device-ram and then excluding device-ram
> > from vhost.
> > 
> > bz: https://bugzilla.redhat.com/show_bug.cgi?id=1779041
> > 
> > Dr. David Alan Gilbert (2):
> >    vhost: Don't pass ram device sections
> >    hyperv/synic: Allocate as ram_device
> > 
> >   hw/hyperv/hyperv.c | 14 ++++++++------
> >   hw/virtio/vhost.c  |  1 +
> >   2 files changed, 9 insertions(+), 6 deletions(-)
> > 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK