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

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Message-ID: <20180910061947.GA30666@infradead.org>
Date:   Sun, 9 Sep 2018 23:19:47 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Jiandi An <jiandi@....com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        "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,
        thomas.lendacky@....com, brijesh.singh@....com, jiandi.an@....com
Subject: Re: [RFC 0/4] Virtio uses DMA API for all devices

On Thu, Sep 06, 2018 at 07:09:09PM -0500, Jiandi An wrote:
> For virtio device we have to pass in iommu_platform=true flag for
> this to set the VIRTIO_F_IOMMU_PLATFORM flag. But for example
> QEMU has the use of iommu_platform attribute disabled for virtio-gpu
> device.  So would also like to move towards not having to specify
> the VIRTIO_F_IOMMU_PLATFORM flag.

Specifying VIRTIO_F_IOMMU_PLATFORM is the right thing for your 
platform given that you can't directly use physical addresses.
Please fix qemu so that virtio-gpu works with VIRTIO_F_IOMMU_PLATFORM.

Also just as I said for the power folks: you should really work with
the qemu folks that VIRTIO_F_IOMMU_PLATFORM (or whatever we call the
properly documented flag) can be set by default, and no pointless
performance overhead is implied by having a sane and simple
implementation.