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

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Date:   Tue, 07 Aug 2018 16:44:01 +1000
From:   Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        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 Mon, 2018-08-06 at 23:27 -0700, Christoph Hellwig wrote:
> On Tue, Aug 07, 2018 at 08:13:56AM +1000, Benjamin Herrenschmidt wrote:
> > It would be indeed ideal if all we had to do was setup some kind of
> > bus_dma_mask on all PCI devices and have virtio automagically insert
> > swiotlb when necessary.
> 
> For 4.20 I plan to remove the swiotlb ops and instead do the bounce
> buffering in the common code, including a direct call to the direct
> ops to avoid retpoline overhead.  For that you still need a flag
> in virtio that instead of blindly working physical addresses it needs
> to be treated like a real device in terms of DMA.

But you will still call the swiotlb infrastructure, right ? IE, I sitll
need to control where/how the swiotlb "pool" is allocated.
> 
> And for powerpc to make use of that I need to get the dma series I
> posted last week reviewed and included, otherwise powerpc will have
> to be excepted (like arm, where rmk didn't like the way the code
> was factored, everything else has already been taken care of).
> 
> https://lists.linuxfoundation.org/pipermail/iommu/2018-July/028989.html

Yes, I saw your series. I'm just back from a week of travel, I plan to
start reviewing it this week if Michael doesn't beat me to it.

Cheers,
Ben.