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

[Jason Wang]

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. 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?


> 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

Then

vector_sel
address
data
pending
mask
unmask

for configuring msi table?

Thanks


> Thanks!
>
> Jing
>
>
> > ?
> >
> > Thanks
> >
> >
> > > > Thanks
> > > >
> > > >
> > > >
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