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

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Date:   Mon, 6 Aug 2018 23:18:14 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        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 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.