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<address@hidden>
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.