linux-kernel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Jason Wang <jasowang@redhat.com>
To: "Liu, Jing2" <jing2.liu@linux.intel.com>,
	Zha Bin <zhabin@linux.alibaba.com>,
	linux-kernel@vger.kernel.org
Cc: virtio-dev@lists.oasis-open.org, slp@redhat.com, mst@redhat.com,
	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: Wed, 12 Feb 2020 17:06:52 +0800	[thread overview]
Message-ID: <44209f3c-613c-3766-ca83-321b77b0f0dd@redhat.com> (raw)
In-Reply-To: <4c19292f-9d25-a859-3dde-6dd5a03fdf0b@linux.intel.com>


On 2020/2/12 上午11:54, Liu, Jing2 wrote:
>
>
> On 2/11/2020 3:40 PM, 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. 
>
> Right, per vq msix can also be mapped by the 2 commands if we want.
>
> The current design benefits for those devices requesting per vq msi 
> that driver
>
> doesn't have to map during setup each queue,
>
> since we define the relationship by default.
>

Well since you've defined the dynamic mapping, having some "default" 
mapping won't help to reduce the complexity but increase it.


>
>> I don't get why you need to introduce MSI_SHARING_MASK which is the 
>> charge of driver instead of device. 
>
> MSI_SHARING_MASK is just for identifying the msi_sharing bit in 
> readl(MsiState) (0x0c4). The device tells whether it wants msi_sharing.
>
> MsiState register: R
>
> le32 {
>     msi_enabled : 1;
>     msi_sharing: 1;
>     reserved : 30;
> };
>

The question is why device want such information.


>
>> 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
>
> Thanks for the advice. :)
>
> Actually when looking into pci, the queue_msix_vector/msix_config is 
> the msi vector index, which is the same as the mmio register MsiVecSel 
> (0x0d0).
>
> So we don't introduce two extra registers for mapping even in sharing 
> mode.
>
> What do you think?
>

I'm not sure I get the point, but I still prefer the separate vector_sel 
from queue_msix_vector.

Btw, Michael propose per vq registers which could also work.

Thanks


>
>>
>> Then
>>
>> vector_sel
>> address
>> data
>> pending
>> mask
>> unmask
>>
>> for configuring msi table?
>
> PCI-like msix table is not introduced to device and instead simply use 
> commands to tell the mask/configure/enable.
>
> Thanks!
>
> Jing
>
>>
>> Thanks
>>
>>
>>> Thanks!
>>>
>>> Jing
>>>
>>>
>>>>
>>>> ?
>>>>
>>>> Thanks
>>>>
>>>>
>>>>>
>>>>>
>>>>>> Thanks
>>>>>>
>>>>>>
>>>>>>
>>>>>> --------------------------------------------------------------------- 
>>>>>>
>>>>>> To unsubscribe, e-mail: virtio-dev-unsubscribe@lists.oasis-open.org
>>>>>> For additional commands, e-mail: 
>>>>>> virtio-dev-help@lists.oasis-open.org
>>>>>>
>>>>>
>>>>
>>


  parent reply	other threads:[~2020-02-12  9:07 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
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 [this message]
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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=44209f3c-613c-3766-ca83-321b77b0f0dd@redhat.com \
    --to=jasowang@redhat.com \
    --cc=chao.p.peng@linux.intel.com \
    --cc=gerry@linux.alibaba.com \
    --cc=jing2.liu@linux.intel.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=mst@redhat.com \
    --cc=qemu-devel@nongnu.org \
    --cc=slp@redhat.com \
    --cc=virtio-dev@lists.oasis-open.org \
    --cc=zhabin@linux.alibaba.com \
    --subject='Re: [virtio-dev] Re: [PATCH v2 4/5] virtio-mmio: add MSI interrupt feature support' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).