Re: [virtio-dev] Re: [PATCH v2 4/5] virtio-mmio: add MSI interrupt feature support

["Liu, Jing2" <jing2.liu@linux.intel.com>]

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On 2/11/2020 12:02 PM, Jason Wang wrote:
>
> On 2020/2/11 上午11:35, Liu, Jing2 wrote:
>>
>> On 2/11/2020 11:17 AM, Jason Wang wrote:
>>>
>>> On 2020/2/10 下午5:05, Zha Bin wrote:
>>>> From: Liu Jiang<gerry@linux.alibaba.com>
>>>>
>>>> Userspace VMMs (e.g. Qemu microvm, Firecracker) take advantage of 
>>>> using
>>>> virtio over mmio devices as a lightweight machine model for modern
>>>> cloud. The standard virtio over MMIO transport layer only supports one
>>>> legacy interrupt, which is much heavier than virtio over PCI transport
>>>> layer using MSI. Legacy interrupt has long work path and causes 
>>>> specific
>>>> VMExits in following cases, which would considerably slow down the
>>>> performance:
>>>>
>>>> 1) read interrupt status register
>>>> 2) update interrupt status register
>>>> 3) write IOAPIC EOI register
>>>>
>>>> We proposed to add MSI support for virtio over MMIO via new feature
>>>> bit VIRTIO_F_MMIO_MSI[1] which increases the interrupt performance.
>>>>
>>>> With the VIRTIO_F_MMIO_MSI feature bit supported, the virtio-mmio MSI
>>>> uses msi_sharing[1] to indicate the event and vector mapping.
>>>> Bit 1 is 0: device uses non-sharing and fixed vector per event 
>>>> mapping.
>>>> Bit 1 is 1: device uses sharing mode and dynamic mapping.
>>>
>>>
>>> I believe dynamic mapping should cover the case of fixed vector?
>>>
>> Actually this bit *aims* for msi sharing or msi non-sharing.
>>
>> It means, when msi sharing bit is 1, device doesn't want vector per 
>> queue
>>
>> (it wants msi vector sharing as name) and doesn't want a high 
>> interrupt rate.
>>
>> So driver turns to !per_vq_vectors and has to do dynamical mapping.
>>
>> So they are opposite not superset.
>>
>> Thanks!
>>
>> Jing
>
>
> I think you need add more comments on the command.
>
> E.g if I want to map vector 0 to queue 1, how do I need to do?
>
> write(1, queue_sel);
> write(0, vector_sel);

That's true. Besides, two commands are used for msi sharing mode,

VIRTIO_MMIO_MSI_CMD_MAP_CONFIG and VIRTIO_MMIO_MSI_CMD_MAP_QUEUE.

"To set up the event and vector mapping for MSI sharing mode, driver 
SHOULD write a valid MsiVecSel followed by 
VIRTIO_MMIO_MSI_CMD_MAP_CONFIG/VIRTIO_MMIO_MSI_CMD_MAP_QUEUE command to 
map the configuration change/selected queue events respectively.  " (See 
spec patch 5/5)

So if driver detects the msi sharing mode, when it does setup vq, writes 
the queue_sel (this already exists in setup vq), vector sel and then 
MAP_QUEUE command to do the queue event mapping.

For msi non-sharing mode, no special action is needed because we make 
the rule of per_vq_vector and fixed relationship.

Correct me if this is not that clear for spec/code comments.

Thanks!

Jing


>
> ?
>
> Thanks
>
>
>>
>>
>>> Thanks
>>>
>>>
>>>
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>>
>

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