From mboxrd@z Thu Jan 1 00:00:00 1970 From: Marc Zyngier Subject: Re: [RFC PATCH] KVM: arm/arm64: Enable direct irqfd MSI injection Date: Sun, 17 Mar 2019 19:35:48 +0000 Message-ID: <86o969z42z.wl-marc.zyngier@arm.com> References: <1552833373-19828-1-git-send-email-yuzenghui@huawei.com> Mime-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue") Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: mst@redhat.com, kvm@vger.kernel.org, "Raslan, KarimAllah" , andre.przywara@arm.com, linux-kernel@vger.kernel.org, pbonzini@redhat.com, kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org To: Zenghui Yu Return-path: In-Reply-To: <1552833373-19828-1-git-send-email-yuzenghui@huawei.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: kvmarm-bounces@lists.cs.columbia.edu Sender: kvmarm-bounces@lists.cs.columbia.edu List-Id: kvm.vger.kernel.org On Sun, 17 Mar 2019 14:36:13 +0000, Zenghui Yu 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 > --- > > 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 > #include > #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.