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?


> 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.
>

The ABI is not as straightforward as PCI did. Why not just reuse the PCI 
layout?

E.g having

queue_sel
queue_msix_vector
msix_config

for configuring map between msi vector and queues/config

Then

vector_sel
address
data
pending
mask
unmask

for configuring msi table?

Thanks


> Thanks!
>
> Jing
>
>
>>
>> ?
>>
>> Thanks
>>
>>
>>>
>>>
>>>> Thanks
>>>>
>>>>
>>>>
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>>>
>>