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

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Date:   Tue, 11 Feb 2020 12:02:50 +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:     virtio-dev@...ts.oasis-open.org, slp@...hat.com, mst@...hat.com,
        qemu-devel@...gnu.org, chao.p.peng@...ux.intel.com,
        gerry@...ux.alibaba.com
Subject: Re: [virtio-dev] Re: [PATCH v2 4/5] virtio-mmio: add MSI interrupt
 feature support


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@...ux.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);

?

Thanks


>
>
>> Thanks
>>
>>
>>
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>>
>