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Re: [PATCH 0/2] exclude hyperv synic sections from vhost
From: |
Dr. David Alan Gilbert |
Subject: |
Re: [PATCH 0/2] exclude hyperv synic sections from vhost |
Date: |
Thu, 9 Jan 2020 12:02:16 +0000 |
User-agent: |
Mutt/1.13.0 (2019-11-30) |
* Jason Wang (address@hidden) wrote:
>
> On 2020/1/8 下午9:53, Dr. David Alan Gilbert (git) wrote:
> > From: "Dr. David Alan Gilbert" <address@hidden>
> >
> > 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 / address@hidden / Manchester, UK
Re: [PATCH 2/2] hyperv/synic: Allocate as ram_device, Roman Kagan, 2020/01/09
Re: [PATCH 0/2] exclude hyperv synic sections from vhost, Vitaly Kuznetsov, 2020/01/08
Re: [PATCH 0/2] exclude hyperv synic sections from vhost, Jason Wang, 2020/01/08
Re: [PATCH 0/2] exclude hyperv synic sections from vhost, Roman Kagan, 2020/01/09