From mboxrd@z Thu Jan  1 00:00:00 1970
From: Christoph Hellwig <hch@infradead.org>
Subject: Re: [RFC 0/4] Virtio uses DMA API for all devices
Date: Mon, 6 Aug 2018 23:18:14 -0700
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To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: robh@kernel.org, srikar@linux.vnet.ibm.com, Benjamin Herrenschmidt <benh@kernel.crashing.org>, Will Deacon <will.deacon@arm.com>, linux-kernel@vger.kernel.org, linuxram@us.ibm.com, virtualization@lists.linux-foundation.org, Christoph Hellwig <hch@infradead.org>, jean-philippe.brucker@arm.com, paulus@samba.org, marc.zyngier@arm.com, mpe@ellerman.id.au, joe@perches.com, robin.murphy@arm.com, david@gibson.dropbear.id.au, linuxppc-dev@lists.ozlabs.org, elfring@users.sourceforge.net, haren@linux.vnet.ibm.com, Anshuman Khandual <khandual@linux.vnet.ibm.com>
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On Tue, Aug 07, 2018 at 12:46:34AM +0300, Michael S. Tsirkin wrote:
> Well we have the RFC for that - the switch to using DMA ops unconditionally isn't
> problematic itself IMHO, for now that RFC is blocked
> by its perfromance overhead for now but Christoph says
> he's trying to remove that for direct mappings,
> so we should hopefully be able to get there in X weeks.

The direct calls to dma_direct_ops aren't going to help you with legacy
virtio, given that virtio is specified to deal with physical addresses,
while dma-direct is not in many cases.

It would however help with the case where qemu always sets the platform
dma flag, as we'd avoid the indirect calls for that.