On 1/29/2020 6:14 PM, Michael S. Tsirkin wrote:
On Tue, Jan 21, 2020 at 09:54:33PM +0800, Jing Liu wrote:
Bit 1 msi_sharing reported in the MsiState register indicates the mapping mode
device uses.
Bit 1 is 0 - device uses MSI non-sharing mode. This indicates vector per event
and
fixed static vectors and events relationship. This fits for devices with a high
interrupt
rate and best performance;
Bit 1 is 1 - device uses MSI sharing mode. This indicates vectors and events
dynamic mapping and fits for devices not requiring a high interrupt rate.
Thanks for reviewing! Sorry for the late reply.
For msi sharing/non-sharing mode, we are trying to define a rule that,
simply using 1 bit to indicate what device wants and what driver should
do. Let me try to explain details.
If it's sharing mode (bit=1), it means device doesn't want vector per
queue or a high interrupt rate.
Driver should meet such request.
So define that, device should support at least 2 msi vectors and driver
should use
VIRTIO_MMIO_MSI_CMD_MAP_CONFIG/VIRTIO_MMIO_MSI_CMD_MAP_QUEUE commands
to map the configuration change/selected queue events.
In implementation, driver will detect the bit=1, turn to !per_vq_vector
and map 1 vector for config
and 1 for all queues to meet the request. (Maybe we need restrict this
in spec too.)
It seems that sharing mode is a superset of non-sharing mode.
I think sharing mode is not a superset, because driver should not map
1:1 which will go against
the request from device that it doesn't want 1:1 (non-sharing mode).
In other words, device chooses the way it wants, a high interrupt rate
with 1:1 or not.
Isn't that right? E.g. with sharing mode drivers
can still avoid sharing if they like.
Drivers should not avoid msi sharing if that bit is 1.
Maybe it should just be a hint to drivers whether to share
interrupts,
The sharing mode bit is the hint to indicate whether to share interrupts. :)
instead of a completely different layout?
Except the bit, no other different register layout is there for such
feature.
Thanks!
Jing
diff --git a/msi-state.c b/msi-state.c
index b1fa0c1..d470be4 100644
--- a/msi-state.c
+++ b/msi-state.c
@@ -1,4 +1,5 @@
le32 {
msi_enabled : 1;
- reserved : 31;
+ msi_sharing: 1;
+ reserved : 30;
};
--
2.7.4
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