Re: [RFC PATCH] KVM: arm/arm64: Enable direct irqfd MSI injection

[Marc Zyngier <marc.zyngier@arm.com>]

On Sun, 17 Mar 2019 14:36:13 +0000,
Zenghui Yu <yuzenghui@huawei.com> 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@huawei.com>
> ---
> 
> 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.

Thanks,

	M.

-- 
Jazz is not dead, it just smell funny.