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Date:   Mon, 6 Aug 2018 23:18:14 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Christoph Hellwig <hch@infradead.org>,
        Will Deacon <will.deacon@arm.com>,
        Anshuman Khandual <khandual@linux.vnet.ibm.com>,
        virtualization@lists.linux-foundation.org,
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        aik@ozlabs.ru, robh@kernel.org, joe@perches.com,
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        srikar@linux.vnet.ibm.com, robin.murphy@arm.com,
        jean-philippe.brucker@arm.com, marc.zyngier@arm.com
Subject: Re: [RFC 0/4] Virtio uses DMA API for all devices
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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.