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Re: [PATCH v2 0/3] exclude hyperv synic sections from vhost

From: Roman Kagan
Subject: Re: [PATCH v2 0/3] exclude hyperv synic sections from vhost
Date: Wed, 15 Jan 2020 12:12:59 +0000

On Mon, Jan 13, 2020 at 05:36:44PM +0000, 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.
> Since they're not normal RAM, and they shouldn't have vhost DMAing
> into them, exclude them from the vhost set.

I still don't think this is correct assessment.  These pages are normal
RAM, perfectly eligible for DMA and what not.

It was a thinko to implement them this way, taking the Hyper-V spec too
literally.  Among the downsides is the excessive consumption of KVM
memslots, and unnecessary large page splits or conflicts with
unsplittable ones.  I'm working on an alternative approach that doesn't
suffer from these issues; struggling to preserve compatibility ATM.


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