linux-kernel - Re: [virtio-dev] Re: [PATCH v1 2/2] virtio-mmio: add features for virtio-mmio specification version 3

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Date:   Fri, 3 Jan 2020 11:24:48 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Liu, Jing2" <jing2.liu@...ux.intel.com>,
        Zha Bin <zhabin@...ux.alibaba.com>,
        linux-kernel@...r.kernel.org
Cc:     mst@...hat.com, slp@...hat.com, virtio-dev@...ts.oasis-open.org,
        gerry@...ux.alibaba.com, jing2.liu@...el.com, chao.p.peng@...el.com
Subject: Re: [virtio-dev] Re: [PATCH v1 2/2] virtio-mmio: add features for
 virtio-mmio specification version 3


On 2020/1/2 下午5:13, Liu, Jing2 wrote:
> [...]
>
>>>>
>>>>> +
>>>>> +/* RO: MSI feature enabled mask */
>>>>> +#define VIRTIO_MMIO_MSI_ENABLE_MASK    0x8000
>>>>> +/* RO: Maximum queue size available */
>>>>> +#define VIRTIO_MMIO_MSI_STATUS_QMASK    0x07ff
>>>>> +/* Reserved */
>>>>> +#define VIRTIO_MMIO_MSI_STATUS_RESERVED    0x7800
>>>>> +
>>>>> +#define VIRTIO_MMIO_MSI_CMD_UPDATE    0x1
>>>>
>>>>
>>>> I believe we need a command to read the number of vectors supported 
>>>> by the device, or 2048 is assumed to be a fixed size here?
>>>
>>> For not bringing much complexity, we proposed vector per queue and 
>>> fixed relationship between events and vectors.
>>
>>
>> It's a about the number of MSIs not the mapping between queues to 
>> MSIs.And it looks to me it won't bring obvious complexity, just need 
>> a register to read the #MSIs. Device implementation may stick to a 
>> fixed size.
>
> Based on that assumption, the device supports #MSIs = #queues + 
> #config. Then driver need not read the register.
>
> We're trying to make such kind of agreement on spec level.


Ok, I get you now.

But still, having fixed number of MSIs is less flexible. E.g:

- for x86, processor can only deal with about 250 interrupt vectors.
- driver may choose to share MSI vectors [1] (which is not merged but we 
will for sure need it)

[1] https://lkml.org/lkml/2014/12/25/169


>
>>
>> Having few pages for a device that only have one queue is kind of a 
>> waste.
>
> Could I ask what's the meaning of few pages here? BTW, we didn't 
> define MSIx-like tables for virtio-mmio.


I thought you're using a fixed size (2048) for each device. But looks not :)

Thanks


>
> Thanks,
>
> Jing
>
>>
>> Thanks
>>
>>
>>>
>>>
>>> So the number of vectors supported by device is equal to the total 
>>> number of vqs and config.
>>>
>>> We will try to explicitly highlight this point in spec for later 
>>> version.
>>>
>>>
>>> Thanks!
>>>
>>> Jing
>>>
>>>>
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>>>>
>>>
>>
>
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