On Sun, 17 Mar 2019 14:36:13 +0000,
Zenghui Yu <yuzenghui@xxxxxxxxxx> wrote:
Currently, IRQFD on arm still uses the deferred workqueue mechanism
to inject interrupts into guest, which will likely lead to a busy
context-switching from/to the kworker thread. This overhead is for
no purpose (only in my view ...) and will result in an interrupt
performance degradation.
Implement kvm_arch_set_irq_inatomic() for arm/arm64 to support direct
irqfd MSI injection, by which we can get rid of the annoying latency.
As a result, irqfd MSI intensive scenarios (e.g., DPDK with high packet
processing workloads) will benefit from it.
Signed-off-by: Zenghui Yu <yuzenghui@xxxxxxxxxx>
---
It seems that only MSI will follow the IRQFD path, did I miss something?
This patch is still under test and sent out for early feedback. If I have
any mis-understanding, please fix me up and let me know. Thanks!
As mentioned by other folks in the thread, this is clearly wrong. The
first thing kvm_inject_msi does is to lock the corresponding ITS using
a mutex. So the "no purpose" bit was a bit too quick.
When doing this kind of work, I suggest you enable lockdep and all the
related checkers. Also, for any optimisation, please post actual
numbers for the relevant benchmarks. Saying "application X will
benefit from it" is meaningless without any actual data.
---
virt/kvm/arm/vgic/trace.h | 22 ++++++++++++++++++++++
virt/kvm/arm/vgic/vgic-irqfd.c | 21 +++++++++++++++++++++
2 files changed, 43 insertions(+)
diff --git a/virt/kvm/arm/vgic/trace.h b/virt/kvm/arm/vgic/trace.h
index 55fed77..bc1f4db 100644
--- a/virt/kvm/arm/vgic/trace.h
+++ b/virt/kvm/arm/vgic/trace.h
@@ -27,6 +27,28 @@
__entry->vcpu_id, __entry->irq, __entry->level)
);
+TRACE_EVENT(kvm_arch_set_irq_inatomic,
+ TP_PROTO(u32 gsi, u32 type, int level, int irq_source_id),
+ TP_ARGS(gsi, type, level, irq_source_id),
+
+ TP_STRUCT__entry(
+ __field( u32, gsi )
+ __field( u32, type )
+ __field( int, level )
+ __field( int, irq_source_id )
+ ),
+
+ TP_fast_assign(
+ __entry->gsi = gsi;
+ __entry->type = type;
+ __entry->level = level;
+ __entry->irq_source_id = irq_source_id;
+ ),
+
+ TP_printk("gsi %u type %u level %d source %d", __entry->gsi,
+ __entry->type, __entry->level, __entry->irq_source_id)
+);
+
#endif /* _TRACE_VGIC_H */
#undef TRACE_INCLUDE_PATH
diff --git a/virt/kvm/arm/vgic/vgic-irqfd.c b/virt/kvm/arm/vgic/vgic-irqfd.c
index 99e026d..4cfc3f4 100644
--- a/virt/kvm/arm/vgic/vgic-irqfd.c
+++ b/virt/kvm/arm/vgic/vgic-irqfd.c
@@ -19,6 +19,7 @@
#include <trace/events/kvm.h>
#include <kvm/arm_vgic.h>
#include "vgic.h"
+#include "trace.h"
/**
* vgic_irqfd_set_irq: inject the IRQ corresponding to the
@@ -105,6 +106,26 @@ int kvm_set_msi(struct kvm_kernel_irq_routing_entry *e,
return vgic_its_inject_msi(kvm, &msi);
}
+/**
+ * kvm_arch_set_irq_inatomic: fast-path for irqfd injection
+ *
+ * Currently only direct MSI injecton is supported.
+ */
+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)
+{
+ int ret;
+
+ trace_kvm_arch_set_irq_inatomic(e->gsi, e->type, level, irq_source_id);
+
+ if (unlikely(e->type != KVM_IRQ_ROUTING_MSI))
+ return -EWOULDBLOCK;
+
+ ret = kvm_set_msi(e, kvm, irq_source_id, level, line_status);
+ return ret;
+}
+
Although we've established that the approach is wrong, maybe we can
look at improving this aspect.
A first approach would be to keep a small cache of the last few
successful translations for this ITS, cache that could be looked-up by
holding a spinlock instead. A hit in this cache could directly be
injected. Any command that invalidates or changes anything (DISCARD,
INV, INVALL, MAPC with V=0, MAPD with V=0, MOVALL, MOVI) should nuke
the cache altogether.
Of course, all of that needs to be quantified.
