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

[Jason Wang <jasowang@redhat.com>]

On 2020/2/11 下午7:58, Michael S. Tsirkin wrote:
> On Tue, Feb 11, 2020 at 03:40:23PM +0800, Jason Wang wrote:
>> On 2020/2/11 下午2:02, Liu, Jing2 wrote:
>>> 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.
>>>
>> So actually the per vq msix could be done through this. I don't get why you
>> need to introduce MSI_SHARING_MASK which is the charge of driver instead of
>> device. The interrupt rate should have no direct relationship with whether
>> it has been shared or not.
>>
>> Btw, you introduce mask/unmask without pending, how to deal with the lost
>> interrupt during the masking then?
> pending can be an internal device register. as long as device
> does not lose interrupts while masked, all's well.


You meant raise the interrupt during unmask automatically?


>
> There's value is being able to say "this queue sends no
> interrupts do not bother checking used notification area".
> so we need way to say that. So I guess an enable interrupts
> register might have some value...
> But besides that, it's enough to have mask/unmask/address/data
> per vq.


Just to check, do you mean "per vector" here?

Thanks


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