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

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Message-ID: <20161114151923.GX2078@8bytes.org>
Date:   Mon, 14 Nov 2016 16:19:24 +0100
From:   Joerg Roedel <joro@...tes.org>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     linux-arm-kernel@...ts.infradead.org, drjones@...hat.com,
        jason@...edaemon.net, kvm@...r.kernel.org, marc.zyngier@....com,
        benh@...nel.crashing.org, punit.agrawal@....com,
        Will Deacon <will.deacon@....com>,
        linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        pranav.sawargaonkar@...il.com, arnd@...db.de, dwmw@...zon.co.uk,
        jcm@...hat.com, tglx@...utronix.de,
        Christoffer Dall <christoffer.dall@...aro.org>,
        eric.auger.pro@...il.com
Subject: Re: Summary of LPC guest MSI discussion in Santa Fe

On Fri, Nov 11, 2016 at 09:05:43AM -0700, Alex Williamson wrote:
> On Fri, 11 Nov 2016 08:50:56 -0700
> Alex Williamson <alex.williamson@...hat.com> wrote:
> > 
> > It's really just a happenstance that we don't map RAM over the x86 MSI
> > range though.  That property really can't be guaranteed once we mix
> > architectures, such as running an aarch64 VM on x86 host via TCG.
> > AIUI, the MSI range is actually handled differently than other DMA
> > ranges, so a iommu_map() overlapping a range that the iommu cannot map
> > should fail just like an attempt to map beyond the address width of the
> > iommu.
> 
> (clarification, this is x86 specific, the MSI controller - interrupt
> remapper - is embedded in the iommu AIUI, so the iommu is actually not
> able to provide DMA translation for this range.

Right, on x86 the MSI range can be covered by page-tables, but those are
ignored by the IOMMU hardware. But what I am trying to say is, that
checking for these ranges happens already on a higher level (in the
dma-api implementations by marking these regions as allocted) so that
there is no need to check for them again in the iommu_map/unmap path.



	Joerg