linux-kernel - Re: Summary of LPC guest MSI discussion in Santa Fe

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Message-ID: <20161110100704.7dd16e2f@t450s.home>
Date:   Thu, 10 Nov 2016 10:07:04 -0700
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Joerg Roedel <joro@...tes.org>
Cc:     Christoffer Dall <christoffer.dall@...aro.org>,
        Don Dutile <ddutile@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Eric Auger <eric.auger@...hat.com>, eric.auger.pro@...il.com,
        marc.zyngier@....com, robin.murphy@....com, tglx@...utronix.de,
        jason@...edaemon.net, linux-arm-kernel@...ts.infradead.org,
        kvm@...r.kernel.org, drjones@...hat.com,
        linux-kernel@...r.kernel.org, pranav.sawargaonkar@...il.com,
        iommu@...ts.linux-foundation.org, punit.agrawal@....com,
        diana.craciun@....com, benh@...nel.crashing.org, arnd@...db.de,
        jcm@...hat.com, dwmw@...zon.co.uk
Subject: Re: Summary of LPC guest MSI discussion in Santa Fe

On Thu, 10 Nov 2016 15:40:07 +0100
Joerg Roedel <joro@...tes.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