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

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Date:   Fri, 27 Jul 2018 16:28:33 +0530
From:   Anshuman Khandual <khandual@...ux.vnet.ibm.com>
To:     Will Deacon <will.deacon@....com>
Cc:     robh@...nel.org, srikar@...ux.vnet.ibm.com, mst@...hat.com,
        aik@...abs.ru, jasowang@...hat.com, linuxram@...ibm.com,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, hch@...radead.org,
        jean-philippe.brucker@....com, paulus@...ba.org,
        marc.zyngier@....com, joe@...ches.com, robin.murphy@....com,
        linuxppc-dev@...ts.ozlabs.org, elfring@...rs.sourceforge.net,
        haren@...ux.vnet.ibm.com, david@...son.dropbear.id.au
Subject: Re: [RFC 0/4] Virtio uses DMA API for all devices

On 07/27/2018 03:28 PM, Will Deacon wrote:
> Hi Anshuman,
> 
> On Fri, Jul 20, 2018 at 09:29:37AM +0530, Anshuman Khandual wrote:
>> This patch series is the follow up on the discussions we had before about
>> the RFC titled [RFC,V2] virtio: Add platform specific DMA API translation
>> for virito devices (https://patchwork.kernel.org/patch/10417371/). There
>> were suggestions about doing away with two different paths of transactions
>> with the host/QEMU, first being the direct GPA and the other being the DMA
>> API based translations.
>>
>> First patch attempts to create a direct GPA mapping based DMA operations
>> structure called 'virtio_direct_dma_ops' with exact same implementation
>> of the direct GPA path which virtio core currently has but just wrapped in
>> a DMA API format. Virtio core must use 'virtio_direct_dma_ops' instead of
>> the arch default in absence of VIRTIO_F_IOMMU_PLATFORM flag to preserve the
>> existing semantics. The second patch does exactly that inside the function
>> virtio_finalize_features(). The third patch removes the default direct GPA
>> path from virtio core forcing it to use DMA API callbacks for all devices.
>> Now with that change, every device must have a DMA operations structure
>> associated with it. The fourth patch adds an additional hook which gives
>> the platform an opportunity to do yet another override if required. This
>> platform hook can be used on POWER Ultravisor based protected guests to
>> load up SWIOTLB DMA callbacks to do the required (as discussed previously
>> in the above mentioned thread how host is allowed to access only parts of
>> the guest GPA range) bounce buffering into the shared memory for all I/O
>> scatter gather buffers to be consumed on the host side.
>>
>> Please go through these patches and review whether this approach broadly
>> makes sense. I will appreciate suggestions, inputs, comments regarding
>> the patches or the approach in general. Thank you.
> I just wanted to say that this patch series provides a means for us to
> force the coherent DMA ops for legacy virtio devices on arm64, which in turn
> means that we can enable the SMMU with legacy devices in our fastmodel
> emulation platform (which is slowly being upgraded to virtio 1.0) without
> hanging during boot. Patch below.
> 
> So:
> 
> Acked-by: Will Deacon <will.deacon@....com>
> Tested-by: Will Deacon <will.deacon@....com>

Thanks Will.