linux-kernel - Re: [PATCH 2/2] virtio: let virtio use DMA API when guest RAM is protected

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Date:   Mon, 24 Feb 2020 17:50:53 +1100
From:   David Gibson <david@...son.dropbear.id.au>
To:     Christoph Hellwig <hch@....de>
Cc:     Halil Pasic <pasic@...ux.ibm.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        linux-s390@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Viktor Mihajlovski <mihajlov@...ux.ibm.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Ram Pai <linuxram@...ibm.com>,
        Thiago Jung Bauermann <bauerman@...ux.ibm.com>,
        "Lendacky, Thomas" <Thomas.Lendacky@....com>,
        Michael Mueller <mimu@...ux.ibm.com>
Subject: Re: [PATCH 2/2] virtio: let virtio use DMA API when guest RAM is
 protected

On Fri, Feb 21, 2020 at 05:36:45PM +0100, Christoph Hellwig wrote:
> On Fri, Feb 21, 2020 at 01:59:15PM +1100, David Gibson wrote:
> > > Hell no.  This is a detail of the platform DMA direct implementation.
> > > Drivers have no business looking at this flag, and virtio finally needs
> > > to be fixed to use the DMA API properly for everything but legacy devices.
> > 
> > So, this patch definitely isn't right as it stands, but I'm struggling
> > to understand what it is you're saying is the right way.
> > 
> > By "legacy devices" I assume you mean pre-virtio-1.0 devices, that
> > lack the F_VERSION_1 feature flag.  Is that right?  Because I don't
> > see how being a legacy device or not relates to use of the DMA API.
> 
> No.   "legacy" is anything that does not set F_ACCESS_PLATFORM.

Hm, I see.

The trouble is I think we can only reasonably call things "legacy"
when essentially all currently in use OSes have support for the new,
better way of doing things.  That is, alas, not really the case for
F_ACCESS_PLATFORM.

> > I *think* what you are suggesting here is that virtio devices that
> > have !F_IOMMU_PLATFORM should have their dma_ops set up so that the
> > DMA API treats IOVA==PA, which will satisfy what the device expects.
> > Then the virtio driver can use the DMA API the same way for both
> > F_IOMMU_PLATFORM and !F_IOMMU_PLATFORM devices.
> 
> No.  Those should just keep using the existing legacy non-dma ops
> case and be done with it.  No changes to that and most certainly
> no new features.
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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