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KVM: Isolate apicv_update_lock and apicv_nr_irq_window_req in a cacheline

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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>
Link: https://patch.msgid.link/20260123224514.2509129-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
This commit is contained in:
Sean Christopherson 2026-01-23 14:45:14 -08:00
parent 5617dddcfa
commit fa78a514d6
1 changed files with 11 additions and 1 deletions
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12
arch/x86/include/asm/kvm_host.h
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@ -1441,13 +1441,23 @@ 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;