Hi Marc,
On 2020/5/27 15:55, Marc Zyngier wrote:
Hi Zenghui,
On 2020-05-27 08:41, Zenghui Yu wrote:
On 2020/5/27 0:11, Marc Zyngier wrote:
On a system that uses SPIs to implement MSIs (as it would be
the case on a GICv2 system exposing a GICv2m to its guests),
we deny the possibility of injecting SPIs on the in-atomic
fast-path.
This results in a very large amount of context-switches
(roughly equivalent to twice the interrupt rate) on the host,
and suboptimal performance for the guest (as measured with
a test workload involving a virtio interface backed by vhost-net).
Given that GICv2 systems are usually on the low-end of the spectrum
performance wise, they could do without the aggravation.
We solved this for GICv3+ITS by having a translation cache. But
SPIs do not need any extra infrastructure, and can be immediately
injected in the virtual distributor as the locking is already
heavy enough that we don't need to worry about anything.
This halves the number of context switches for the same workload.
Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
---
arch/arm64/kvm/vgic/vgic-irqfd.c | 20 ++++++++++++++++----
arch/arm64/kvm/vgic/vgic-its.c | 3 +--
2 files changed, 17 insertions(+), 6 deletions(-)
diff --git a/arch/arm64/kvm/vgic/vgic-irqfd.c
b/arch/arm64/kvm/vgic/vgic-irqfd.c
index d8cdfea5cc96..11a9f81115ab 100644
--- a/arch/arm64/kvm/vgic/vgic-irqfd.c
+++ b/arch/arm64/kvm/vgic/vgic-irqfd.c
@@ -107,15 +107,27 @@ int kvm_arch_set_irq_inatomic(struct
kvm_kernel_irq_routing_entry *e,
struct kvm *kvm, int irq_source_id, int level,
bool line_status)
... and you may also need to update the comment on top of it to
reflect this change.
/**
* kvm_arch_set_irq_inatomic: fast-path for irqfd injection
*
* Currently only direct MSI injection is supported.
*/
As far as I can tell, it is still valid (at least from the guest's
perspective). You could in practice use that to deal with level
interrupts, but we only inject the rising edge on this path, never
the falling edge. So effectively, this is limited to edge interrupts,
which is mostly MSIs.
Oops... I had wrongly mixed MSI with the architecture-defined LPI, and
was think that we should add something like "direct SPI injection is
also supported now". Sorry.
Unless you are thinking of something else which I would have missed?
No, please ignore the noisy.
Thanks,
Zenghui
