Force apicv_update_lock and apicv_nr_irq_window_req to reside in their own
cacheline to avoid generating significant contention due to false sharing
when KVM is contantly creating IRQ windows. E.g. apicv_inhibit_reasons is
read on every VM-Enter; disabled_exits is read on page faults, on PAUSE
exits, if a vCPU is scheduled out, etc.; kvmclock_offset is read every time
a vCPU needs to refresh kvmclock, and so on and so forth.
Isolating the write-mostly fields from all other (read-mostly) fields
improves performance by 7-8% when running netperf TCP_RR between two guests
on the same physical host when using an in-kernel PIT in re-inject mode.
Reported-by: Naveen N Rao (AMD) <naveen@kernel.org>
Closes: https://lore.kernel.org/all/yrxhngndj37edud6tj5y3vunaf7nirwor4n63yf4275wdocnd3@c77ujgialc6r
Tested-by: Naveen N Rao (AMD) <naveen@kernel.org>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/include/asm/kvm_host.h | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index b08baeff98b2..8a9f797b6a68 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1435,13 +1435,23 @@ struct kvm_arch {
bool apic_access_memslot_enabled;
bool apic_access_memslot_inhibited;
+ /*
+ * Force apicv_update_lock and apicv_nr_irq_window_req to reside in a
+ * dedicated cacheline. They are write-mostly, whereas most everything
+ * else in kvm_arch is read-mostly. Note that apicv_inhibit_reasons is
+ * read-mostly: toggling VM-wide inhibits is rare; _checking_ for
+ * inhibits is common.
+ */
+ ____cacheline_aligned
/*
* Protects apicv_inhibit_reasons and apicv_nr_irq_window_req (with an
* asterisk, see kvm_inc_or_dec_irq_window_inhibit() for details).
*/
struct rw_semaphore apicv_update_lock;
- unsigned long apicv_inhibit_reasons;
atomic_t apicv_nr_irq_window_req;
+ ____cacheline_aligned
+
+ unsigned long apicv_inhibit_reasons;
gpa_t wall_clock;
--
2.52.0.457.g6b5491de43-goog