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

[Zenghui Yu]

Hi all,

On 2019/3/18 3:35, Marc Zyngier wrote:
> 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.

Thanks for all of your explanations, especially for Marc's suggestions!
It took me long time to figure out my mistakes, since I am not very
familiar with the locking stuff. Now I have to apologize for my noise.

As for the its-translation-cache code (a really good news to us), we
have a rough look at it and start testing now!


thanks,

zenghui

> Thanks,
>
> 	M.