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

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Feb 12, 2020 at 11:54:53AM +0800, 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.

Point being to save some exits for configuration? How much does it
save? IMHO we need to find a way to drastically simplify this interface,
to cut down the new LOC to at most 100-200, proportionally to the
performance gain it gives.


> 
>     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 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?
> 
> 
> 
>     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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