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:   Thu, 2 Jan 2020 17:13:28 +0800
From:   "Liu, Jing2" <jing2.liu@...ux.intel.com>
To:     Jason Wang <jasowang@...hat.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

[...]

>>>
>>>> +
>>>> +/* 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.

>
> 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.

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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>>>
>>
>