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From: "Michael S. Tsirkin" <mst@redhat.com>
To: Jason Wang <jasowang@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 09:00:21 -0500	[thread overview]
Message-ID: <20200211090003-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <fdb19ef4-2003-6c6f-5c6f-c757a6320130@redhat.com>

On Tue, Feb 11, 2020 at 08:18:54PM +0800, Jason Wang wrote:
> 
> 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
> 

masking should be per vq too.

-- 
MST


  reply	other threads:[~2020-02-11 14:00 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 [this message]
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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