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

From: Roman Kagan
Subject: Re: [PATCH 0/2] exclude hyperv synic sections from vhost
Date: Thu, 9 Jan 2020 11:53:56 +0000

On Wed, Jan 08, 2020 at 01:53:51PM +0000, Dr. David Alan Gilbert (git) wrote:
> 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.
> Since they're not normal RAM, and they shouldn't have vhost DMAing
> into them, exclude them from the vhost set.

But they *are* normal RAM.  If the guest driver sets up the device to
DMA to a SynIC page so be it, and the guest deserves what it gets.

> 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

I was pointed a while back by Vitaly at
https://bugs.launchpad.net/qemu/+bug/1811533 which appeared to be the
same issue, but failed to reproduce the problem.  Can you please provide
some more detail as to how it's triggered?


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