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

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Date:   Fri, 3 Aug 2018 10:41:41 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>
Cc:     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,
        benh@...nel.crashing.org, mpe@...erman.id.au, hch@...radead.org,
        linuxram@...ibm.com, haren@...ux.vnet.ibm.com, paulus@...ba.org,
        srikar@...ux.vnet.ibm.com
Subject: Re: [RFC 0/4] Virtio uses DMA API for all devices



On 2018年08月03日 04:55, Michael S. Tsirkin wrote:
> 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.
> Jason did some work on profiling this. Unfortunately he reports
> about 4% extra overhead from this switch on x86 with no vIOMMU.

The test is rather simple, just run pktgen (pktgen_sample01_simple.sh) 
in guest and measure PPS on tap on host.

Thanks

>
> I expect he's writing up the data in more detail, but
> just wanted to let you know this would be one more
> thing to debug before we can just switch to DMA APIs.
>
>
>> Anshuman Khandual (4):
>>    virtio: Define virtio_direct_dma_ops structure
>>    virtio: Override device's DMA OPS with virtio_direct_dma_ops selectively
>>    virtio: Force virtio core to use DMA API callbacks for all virtio devices
>>    virtio: Add platform specific DMA API translation for virito devices
>>
>>   arch/powerpc/include/asm/dma-mapping.h |  6 +++
>>   arch/powerpc/platforms/pseries/iommu.c |  6 +++
>>   drivers/virtio/virtio.c                | 72 ++++++++++++++++++++++++++++++++++
>>   drivers/virtio/virtio_pci_common.h     |  3 ++
>>   drivers/virtio/virtio_ring.c           | 65 +-----------------------------
>>   5 files changed, 89 insertions(+), 63 deletions(-)
>>
>> -- 
>> 2.9.3