[PATCH 0/5] KVM: arm64: Accelerate lookup of vcpus by MPIDR values

[Marc Zyngier]

Xu Zhao recently reported[1] that sending SGIs on large VMs was slower
than expected, specially if targeting vcpus that have a high vcpu
index. They root-caused it to the way we walk the vcpu xarray in the
search of the correct MPIDR, one vcpu at a time, which is of course
grossly inefficient.

The solution they proposed was, unfortunately, less than ideal, but I
was "nerd snipped" into doing something about it.

The main idea is to build a small hash table of MPIDR to vcpu
mappings, using the fact that most of the time, the MPIDR values only
use a small number of significant bits and that we can easily compute
a compact index from it. Once we have that, accelerating vcpu lookup
becomes pretty cheap, and we can in turn make SGIs great again.

It must be noted that since the MPIDR values are controlled by
userspace, it isn't always possible to allocate the hash table
(userspace could build a 32 vcpu VM and allocate one bit of affinity
to each of them, making all the bits significant). We thus always have
an iterative fallback -- if it hurts, don't do that.

Performance wise, this is very significant: using the KUT micro-bench
test with the following patch (always IPI-ing the last vcpu of the VM)
and running it with large number of vcpus shows a large improvement
(from 3832ns to 2593ns for a 64 vcpu VM, a 32% reduction, measured on
an Ampere Altra). I expect that IPI-happy workloads could benefit from
this.

Thanks,

	M.

[1] https://lore.kernel.org/r/20230825015811.5292-1-zhaoxu.35@bytedance.com

diff --git a/arm/micro-bench.c b/arm/micro-bench.c
index bfd181dc..f3ac3270 100644
--- a/arm/micro-bench.c
+++ b/arm/micro-bench.c
@@ -88,7 +88,7 @@ static bool test_init(void)
 
 	irq_ready = false;
 	gic_enable_defaults();
-	on_cpu_async(1, gic_secondary_entry, NULL);
+	on_cpu_async(nr_cpus - 1, gic_secondary_entry, NULL);
 
 	cntfrq = get_cntfrq();
 	printf("Timer Frequency %d Hz (Output in microseconds)\n", cntfrq);
@@ -157,7 +157,7 @@ static void ipi_exec(void)
 
 	irq_received = false;
 
-	gic_ipi_send_single(1, 1);
+	gic_ipi_send_single(1, nr_cpus - 1);
 
 	while (!irq_received && tries--)
 		cpu_relax();


Marc Zyngier (5):
  KVM: arm64: Simplify kvm_vcpu_get_mpidr_aff()
  KVM: arm64: Build MPIDR to vcpu index cache at runtime
  KVM: arm64: Fast-track kvm_mpidr_to_vcpu() when mpidr_data is
    available
  KVM: arm64: vgic-v3: Refactor GICv3 SGI generation
  KVM: arm64: vgic-v3: Optimize affinity-based SGI injection

 arch/arm64/include/asm/kvm_emulate.h |   2 +-
 arch/arm64/include/asm/kvm_host.h    |  28 ++++++
 arch/arm64/kvm/arm.c                 |  66 +++++++++++++
 arch/arm64/kvm/vgic/vgic-mmio-v3.c   | 142 ++++++++++-----------------
 4 files changed, 148 insertions(+), 90 deletions(-)

-- 
2.34.1