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

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Date:   Sun, 5 Aug 2018 00:25:06 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.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 Sun, Aug 05, 2018 at 03:09:55AM +0300, Michael S. Tsirkin wrote:
> So in this case however I'm not sure what exactly do we want to add. It
> seems that from point of view of the device, there is nothing special -
> it just gets a PA and writes there.  It also seems that guest does not
> need to get any info from the device either. Instead guest itself needs
> device to DMA into specific addresses, for its own reasons.
> 
> It seems that the fact that within guest it's implemented using a bounce
> buffer and that it's easiest to do by switching virtio to use the DMA API
> isn't something virtio spec concerns itself with.

And that is exactly what we added bus_dma_mask for - the case where
the device itself has not limitation (or a bigger limitation), but
the platform limits the accessible dma ranges.  One typical case is
a PCIe root port that is only connected to the CPU through an
interconnect that is limited to 32 address bits for example.