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

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Date:   Thu, 2 Aug 2018 23:52:10 +0300
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>
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, Aug 02, 2018 at 12:53:41PM -0500, Benjamin Herrenschmidt wrote:
> 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.
> 

Right but as I tried to say doing that brings us to a bunch of issues
with using DMA APIs in virtio. Put simply DMA APIs weren't designed for
guest to hypervisor communication.

When we do (as is the case with PLATFORM_IOMMU right now) this adds a
bunch of overhead which we need to get rid of if we are to switch to
PLATFORM_IOMMU by default.  We need to fix that.

-- 
MST