linux-kernel - Re: [RFC 0/4] Virtio uses DMA API for all devices

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Date:   Thu, 02 Aug 2018 12:53:41 -0500
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Will Deacon <will.deacon@....com>,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        aik@...abs.ru, robh@...nel.org, joe@...ches.com,
        elfring@...rs.sourceforge.net, david@...son.dropbear.id.au,
        jasowang@...hat.com, mpe@...erman.id.au, linuxram@...ibm.com,
        haren@...ux.vnet.ibm.com, paulus@...ba.org,
        srikar@...ux.vnet.ibm.com, robin.murphy@....com,
        jean-philippe.brucker@....com, marc.zyngier@....com
Subject: Re: [RFC 0/4] Virtio uses DMA API for all devices

On Thu, 2018-08-02 at 20:19 +0300, Michael S. Tsirkin wrote:
> 
> I see. So yes, given that device does not know or care, using
> virtio features is an awkward fit.
> 
> So let's say as a quick fix for you maybe we could generalize the
> xen_domain hack, instead of just checking xen_domain check some static
> branch.  Then teach xen and others to enable that.>

> OK but problem then becomes this: if you do this and virtio device appears
> behind a vIOMMU and it does not advertize the IOMMU flag, the
> code will try to use the vIOMMU mappings and fail.
>
> It does look like even with trick above, you need a special version of
> DMA ops that does just swiotlb but not any of the other things DMA API
> might do.
> 
> Thoughts?

Yes, this is the purpose of Anshuman original patch (I haven't looked
at the details of the patch in a while but that's what I told him to
implement ;-) :

 - Make virtio always use DMA ops to simplify the code path (with a set
of "transparent" ops for legacy)

 and

 -  Provide an arch hook allowing us to "override" those "transparent"
DMA ops with some custom ones that do the appropriate swiotlb gunk.

Cheers,
Ben.