[PATCH] arm64: archrandom: avoid trapping ID register read in __cpu_has_rng()

[Aman Priyadarshi <amanp@apple.com>]

__cpu_has_rng() has an early-boot fallback, taken before the
ARM64_HAS_RNG alternative is patched, that calls
this_cpu_has_cap(ARM64_HAS_RNG). With SCOPE_LOCAL_CPU that resolves the
capability by reading ID_AA64ISAR0_EL1 directly from hardware via
__read_sysreg_by_encoding(), on every invocation.

Until the CRNG is seeded, crng_make_state() routes every get_random_*()
through extract_entropy(), which drains architectural entropy via
arch_get_random_seed_longs()/arch_get_random_longs() and so calls
__cpu_has_rng() several times per request. On a direct (non-EFI) boot
there is no bootloader seed, so the CRNG stays unseeded for much of
boot and essentially every early randomness consumer takes this path.

Under virtualization this is costly: the hypervisor traps guest
accesses to the ID registers (HCR_EL2.TID3), making each read a vmexit,
producing ~200k trapped ID_AA64ISAR0_EL1 reads during boot.

The register value is invariant, so read the sanitised feature register
instead. read_sanitised_ftr_reg() returns the cached value from
arm64_ftr_regs[] with no sysreg access, and hence no trap. That array
is populated by cpuinfo_store_boot_cpu() in smp_prepare_boot_cpu(),
before the first early RNG use in random_init_early(), so it is always
valid here.

With this change the trapped reads drop from ~200k to handful number of
times, and the boot time drops roughly by 6.3% in the test environment.

Fixes: 2c03e16f4499 ("random: remove early archrandom abstraction")
Cc: Jason A. Donenfeld <Jason@zx2c4.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Aman Priyadarshi <amanp@apple.com>
---
 arch/arm64/include/asm/archrandom.h | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/archrandom.h b/arch/arm64/include/asm/archrandom.h
index 8babfbe31f95..8067e9a35641 100644
--- a/arch/arm64/include/asm/archrandom.h
+++ b/arch/arm64/include/asm/archrandom.h
@@ -61,8 +61,22 @@ static inline bool __arm64_rndrrs(unsigned long *v)
 
 static __always_inline bool __cpu_has_rng(void)
 {
-	if (unlikely(!system_capabilities_finalized() && !preemptible()))
-		return this_cpu_has_cap(ARM64_HAS_RNG);
+	if (unlikely(!system_capabilities_finalized() && !preemptible())) {
+		/*
+		 * Until the ARM64_HAS_RNG alternative is patched we can't use
+		 * the static-branch form, so consult the feature register
+		 * directly. Don't use this_cpu_has_cap() here: it reads
+		 * ID_AA64ISAR0_EL1 from hardware on every call, under
+		 * virtualization each ID register read traps to the hypervisor
+		 * (HCR_EL2.TID3) -- producing a storm of vmexits during boot.
+		 * The sanitised value is cached in memory.
+		 */
+		u64 isar0 = read_sanitised_ftr_reg(SYS_ID_AA64ISAR0_EL1);
+
+		return cpuid_feature_extract_unsigned_field(isar0,
+				ID_AA64ISAR0_EL1_RNDR_SHIFT) >=
+			ID_AA64ISAR0_EL1_RNDR_IMP;
+	}
 	return alternative_has_cap_unlikely(ARM64_HAS_RNG);
 }
 
-- 
2.54.0 (Apple Git-156)