[PATCH v2] arm64: Avoid repeated AA64MMFR1_EL1 register read on pagefault path

[Gabriel Krisman Bertazi]

Accessing AA64MMFR1_EL1 is expensive in KVM guests, since it is emulated
in the hypervisor.  In fact, ARM documentation mentions some feature
registers are not supposed to be accessed frequently by the OS, and
therefore should be emulated for guests [1].

Commit 0388f9c74330 ("arm64: mm: Implement
arch_wants_old_prefaulted_pte()") introduced a read of this register in
the page fault path.  But, even when the feature of setting faultaround
pages with the old flag is disabled for a given cpu, we are still paying
the cost of checking the register on every pagefault. This results in an
explosion of vmexit events in KVM guests, which directly impacts the
performance of virtualized workloads.  For instance, running kernbench
yields a 15% increase in system time solely due to the increased vmexit
cycles.

This patch avoids the extra cost by using the sanitized cached value.
It should be safe to do so, since this register mustn't change for a
given cpu.

[1] https://developer.arm.com/-/media/Arm%20Developer%20Community/PDF/Learn%20the%20Architecture/Armv8-A%20virtualization.pdf?revision=a765a7df-1a00-434d-b241-357bfda2dd31

Signed-off-by: Gabriel Krisman Bertazi <krisman at suse.de>

---
Changes since v1:
  - Rely on read_sanitised_ftr_reg instead of static var (will)
---
 arch/arm64/include/asm/cpufeature.h | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
index 03d1c9d7af82..3d1a5677321e 100644
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h
@@ -864,7 +864,11 @@ static inline bool cpu_has_hw_af(void)
 	if (!IS_ENABLED(CONFIG_ARM64_HW_AFDBM))
 		return false;
 
-	mmfr1 = read_cpuid(ID_AA64MMFR1_EL1);
+	/*
+	 * Use cached version to avoid emulated msr operation on KVM
+	 * guests.
+	 */
+	mmfr1 = read_sanitised_ftr_reg(SYS_ID_AA64MMFR1_EL1);
 	return cpuid_feature_extract_unsigned_field(mmfr1,
 						ID_AA64MMFR1_EL1_HAFDBS_SHIFT);
 }
-- 
2.35.3