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

[Michael S. Tsirkin]

On Wed, Feb 12, 2020 at 05:06:52PM +0800, Jason Wang wrote:
> 
> On 2020/2/12 äå11:54, Liu, Jing2 wrote:
> > 
> > 
> > On 2/11/2020 3:40 PM, Jason Wang wrote:
> > > 
> > > On 2020/2/11 äå2:02, Liu, Jing2 wrote:
> > > > 
> > > > 
> > > > On 2/11/2020 12:02 PM, Jason Wang wrote:
> > > > > 
> > > > > On 2020/2/11 äå11:35, Liu, Jing2 wrote:
> > > > > > 
> > > > > > On 2/11/2020 11:17 AM, Jason Wang wrote:
> > > > > > > 
> > > > > > > On 2020/2/10 äå5:05, Zha Bin wrote:
> > > > > > > > From: Liu Jiang<gerry@xxxxxxxxxxxxxxxxx>
> > > > > > > > 
> > > > > > > > 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 {
> > ÂÂÂ msi_enabled : 1;
> > ÂÂÂ msi_sharing: 1;
> > ÂÂÂ 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
> 

Right and I'd even ask a question: do we need shared MSI at all?
Is it somehow better than legacy interrupt? And why?
Performance numbers please.

> > 
> > > 
> > > 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
> > > > > > > 
> > > > > > > 
> > > > > > > 
> > > > > > > ---------------------------------------------------------------------
> > > > > > > 
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> > > > > > 
> > > > > 
> > >