Re: [PATCH v1 2/2] virtio-mmio: add features for virtio-mmio specification version 3

[Jason Wang <jasowang@redhat.com>]

On 2019/12/25 äå11:20, Liu, Jiang wrote:
> > On Dec 25, 2019, at 6:20 PM, Jason Wang <jasowang@redhat.com> wrote:
> >
> >
> > On 2019/12/25 äå10:50, 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 update virtio over MMIO to version 3[1] to add the
> > > following new features and enhance the performance.
> > >
> > > 1) Support Message Signaled Interrupt(MSI), which increases the
> > >     interrupt performance for virtio multi-queue devices
> > > 2) Support per-queue doorbell, so the guest kernel may directly write
> > >     to the doorbells provided by virtio devices.
> > >
> > > The following is the network tcp_rr performance testing report, tested
> > > with virtio-pci device, vanilla virtio-mmio device and patched
> > > virtio-mmio device (run test 3 times for each case):
> > >
> > > 	netperf -t TCP_RR -H 192.168.1.36 -l 30 -- -r 32,1024
> > >
> > > 		Virtio-PCI    Virtio-MMIO   Virtio-MMIO(MSI)
> > > 	trans/s	    9536	6939		9500
> > > 	trans/s	    9734	7029		9749
> > > 	trans/s	    9894	7095		9318
> > >
> > > [1] https://lkml.org/lkml/2019/12/20/113
> >
> > Thanks for the patch. Two questions after a quick glance:
> >
> > 1) In PCI we choose to support MSI-X instead of MSI for having extra flexibility like alias, independent data and address (e.g for affinity) . Any reason for not start from MSI-X? E.g having MSI-X table and PBA (both of which looks pretty independent).
> Hi Jason,
> 	Thanks for reviewing patches on Christmas Day:)
> 	The PCI MSI-x has several advantages over PCI MSI, mainly
> 1) support 2048 vectors, much more than 32 vectors supported by MSI.
> 2) dedicated address/data for each vector,
> 3) per vector mask/pending bits.
> The proposed MMIO MSI extension supports both 1) and 2),


Aha right, I mis-read the patch. But more questions comes:

1) The association between vq and MSI-X vector is fixed. This means it 
can't work for a device that have more than 2047 queues. We probably 
need something similar to virtio-pci to allow a dynamic association.
2) The mask and unmask control is missed


>   but the extension doesnât support 3) because
> we noticed that the Linux virtio subsystem doesnât really make use of interrupt masking/unmasking.


Not directly used but masking/unmasking is widely used in irq subsystem 
which allows lots of optimizations.


> On the other hand, we want to simplify VMM implementations as simple as possible, and mimicking the PCI MSI-x
> will cause some complexity to VMM implementations.


I agree to simplify VMM implementation, but it looks to me introducing 
masking/pending won't cost too much code in the VMM implementation. Just 
new type of command for VIRTIO_MMIO_MSI_COMMAND.

Thanks


> > 2) It's better to split notify_multiplexer out of MSI support to ease the reviewers (apply to spec patch as well)
> Great suggestion, we will try to split the patch.
>
> Thanks,
> Gerry
>
> > Thanks