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From: Jason Wang <jasowang@redhat.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: virtio-dev@lists.oasis-open.org,
	Zha Bin <zhabin@linux.alibaba.com>,
	slp@redhat.com, "Liu, Jing2" <jing2.liu@linux.intel.com>,
	linux-kernel@vger.kernel.org, qemu-devel@nongnu.org,
	chao.p.peng@linux.intel.com, gerry@linux.alibaba.com
Subject: Re: [virtio-dev] Re: [PATCH v2 4/5] virtio-mmio: add MSI interrupt feature support
Date: Tue, 11 Feb 2020 20:18:54 +0800	[thread overview]
Message-ID: <fdb19ef4-2003-6c6f-5c6f-c757a6320130@redhat.com> (raw)
In-Reply-To: <20200211070523-mutt-send-email-mst@kernel.org>


On 2020/2/11 下午8:08, Michael S. Tsirkin wrote:
> On Tue, Feb 11, 2020 at 08:04:24PM +0800, Jason Wang wrote:
>> 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?
>>
>
> yes - that's also what pci does.
>
> the guest visible pending bit is partially implemented in qemu
> as per pci spec but it's unused.


Ok.


>
>>> 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
>>
> No, per VQ. An indirection VQ -> vector -> address/data isn't
> necessary for PCI either, but that ship has sailed.


Yes, it can work but it may bring extra effort when you want to mask a 
vector which is was shared by a lot of queues.

Thanks

>


  reply	other threads:[~2020-02-11 12:19 UTC|newest]

Thread overview: 40+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-10  9:05 [PATCH v2 0/5] virtio mmio specification enhancement Zha Bin
2020-02-10  9:05 ` [PATCH v2 1/5] virtio-mmio: add notify feature for per-queue Zha Bin
2020-02-11 10:50   ` Michael S. Tsirkin
2020-02-11 11:33   ` Michael S. Tsirkin
2020-02-12  3:39     ` Jason Wang
2020-02-12  8:18       ` Michael S. Tsirkin
2020-02-12  8:53         ` Jason Wang
2020-02-12  9:33           ` Jason Wang
2020-02-12  9:55             ` Michael S. Tsirkin
2020-02-13  3:38               ` Jason Wang
2020-02-10  9:05 ` [PATCH v2 2/5] virtio-mmio: refactor common functionality Zha Bin
2020-02-11 11:19   ` Michael S. Tsirkin
2020-02-12  2:58     ` [virtio-dev] " Liu, Jing2
2020-02-12  7:29       ` Michael S. Tsirkin
2020-02-10  9:05 ` [PATCH v2 3/5] virtio-mmio: create a generic MSI irq domain Zha Bin
2020-02-11 11:16   ` Michael S. Tsirkin
2020-02-12  7:40   ` Michael S. Tsirkin
2020-02-10  9:05 ` [PATCH v2 4/5] virtio-mmio: add MSI interrupt feature support Zha Bin
2020-02-11  3:17   ` Jason Wang
2020-02-11  3:35     ` [virtio-dev] " Liu, Jing2
2020-02-11  4:02       ` Jason Wang
     [not found]         ` <5522f205-207b-b012-6631-3cc77dde3bfe@linux.intel.com>
2020-02-11  7:40           ` Jason Wang
2020-02-11 11:58             ` Michael S. Tsirkin
2020-02-11 12:04               ` Jason Wang
2020-02-11 12:08                 ` Michael S. Tsirkin
2020-02-11 12:18                   ` Jason Wang [this message]
2020-02-11 14:00                     ` Michael S. Tsirkin
2020-02-12  9:03                       ` Jason Wang
2020-02-12  9:15                         ` Michael S. Tsirkin
     [not found]             ` <4c19292f-9d25-a859-3dde-6dd5a03fdf0b@linux.intel.com>
2020-02-12  7:33               ` Michael S. Tsirkin
2020-02-12  9:06               ` Jason Wang
2020-02-12  9:16                 ` Michael S. Tsirkin
2020-02-13  3:40                   ` Jason Wang
2020-02-11 11:21       ` Michael S. Tsirkin
2020-02-11 11:11   ` Michael S. Tsirkin
2020-02-10  9:05 ` [PATCH v2 5/5] x86: virtio-mmio: support virtio-mmio with MSI for x86 Zha Bin
2020-02-11 11:14   ` Michael S. Tsirkin
2020-02-10 11:44 ` [PATCH v2 0/5] virtio mmio specification enhancement Michael S. Tsirkin
2020-02-11 16:05   ` Chao Peng
2020-02-11 10:57     ` Michael S. Tsirkin

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