Date: Thu, 10 Nov 2016 10:07:04 -0700
From: Alex Williamson <alex.williamson@redhat.com>
To: Joerg Roedel <joro@8bytes.org>
Cc: Christoffer Dall <christoffer.dall@linaro.org>,
        Don Dutile <ddutile@redhat.com>, Will Deacon <will.deacon@arm.com>,
        Eric Auger <eric.auger@redhat.com>, eric.auger.pro@gmail.com,
        marc.zyngier@arm.com, robin.murphy@arm.com, tglx@linutronix.de,
        jason@lakedaemon.net, linux-arm-kernel@lists.infradead.org,
        kvm@vger.kernel.org, drjones@redhat.com, linux-kernel@vger.kernel.org,
        pranav.sawargaonkar@gmail.com, iommu@lists.linux-foundation.org,
        punit.agrawal@arm.com, diana.craciun@nxp.com, benh@kernel.crashing.org,
        arnd@arndb.de, jcm@redhat.com, dwmw@amazon.co.uk
Subject: Re: Summary of LPC guest MSI discussion in Santa Fe
Message-ID: <20161110100704.7dd16e2f@t450s.home>
In-Reply-To: <20161110144007.GC2078@8bytes.org>
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On Thu, 10 Nov 2016 15:40:07 +0100
Joerg Roedel <joro@8bytes.org> wrote:

> On Wed, Nov 09, 2016 at 01:01:14PM -0700, Alex Williamson wrote:
> > Well, it's not like QEMU or libvirt stumbling through sysfs to figure
> > out where holes could be in order to instantiate a VM with matching
> > holes, just in case someone might decide to hot-add a device into the
> > VM, at some point, and hopefully they don't migrate the VM to another
> > host with a different layout first, is all that much less disgusting or
> > foolproof. It's just that in order to dynamically remove a page as a
> > possible DMA target we require a paravirt channel, such as a balloon
> > driver that's able to pluck a specific page.  In some ways it's
> > actually less disgusting, but it puts some prerequisites on
> > enlightening the guest OS.  Thanks,  
> 
> I think it is much simpler if libvirt/qemu just go through all
> potentially assignable devices on a system and pre-exclude any addresses
> from guest RAM beforehand, rather than doing something like this with
> paravirt/ballooning when a device is hot-added. There is no guarantee
> that you can take a page away from a linux-guest.

Right, I'm not advocating a paravirt ballooning approach, just pointing
out that they're both terrible, just in different ways.  Thanks,

Alex