[PATCH v2] target/arm: update the cortex-a15 MIDR to latest rev

[PATCH v2] target/arm: update the cortex-a15 MIDR to latest rev

[Alex Bennée]

QEMU doesn't model micro-architectural details which includes most
chip errata. The ARM_ERRATA_798181 work around in the Linux
kernel (see erratum_a15_798181_init) currently detects QEMU's
cortex-a15 as broken and triggers additional expensive TLB flushes as
a result.

Change the MIDR to report what the latest silicon would (r4p0). We
explicitly set the IMPDEF revidr bits to 0 because we don't need to
set anything other than the silicon revision to indicate these flushes
are not needed. This cuts about 5s from my Debian kernel boot with the
latest 6.0rc1 kernel (29s->24s).

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Cc: Arnd Bergmann <arnd@linaro.org>
Cc: Anders Roxell <anders.roxell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Anders Roxell <anders.roxell@linaro.org>
Message-Id: <20220906172257.2776521-1-alex.bennee@linaro.org>

---
v2
  - set revdir=0 instead of 0x200, update commit message
---
 target/arm/cpu_tcg.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/target/arm/cpu_tcg.c b/target/arm/cpu_tcg.c
index 98b5ba2160..60ff539fa1 100644
--- a/target/arm/cpu_tcg.c
+++ b/target/arm/cpu_tcg.c
@@ -592,7 +592,9 @@ static void cortex_a15_initfn(Object *obj)
     set_feature(&cpu->env, ARM_FEATURE_EL3);
     set_feature(&cpu->env, ARM_FEATURE_PMU);
     cpu->kvm_target = QEMU_KVM_ARM_TARGET_CORTEX_A15;
-    cpu->midr = 0x412fc0f1;
+    /* r4p0 cpu, not requiring expensive tlb flush errata */
+    cpu->midr = 0x414fc0f0;
+    cpu->revidr = 0x0;
     cpu->reset_fpsid = 0x410430f0;
     cpu->isar.mvfr0 = 0x10110222;
     cpu->isar.mvfr1 = 0x11111111;
-- 
2.34.1

Re: [PATCH v2] target/arm: update the cortex-a15 MIDR to latest rev

[Peter Maydell]

On Mon, 10 Oct 2022 at 16:32, Alex Bennée <alex.bennee@linaro.org> wrote:
>
> QEMU doesn't model micro-architectural details which includes most
> chip errata. The ARM_ERRATA_798181 work around in the Linux
> kernel (see erratum_a15_798181_init) currently detects QEMU's
> cortex-a15 as broken and triggers additional expensive TLB flushes as
> a result.
>
> Change the MIDR to report what the latest silicon would (r4p0). We
> explicitly set the IMPDEF revidr bits to 0 because we don't need to
> set anything other than the silicon revision to indicate these flushes
> are not needed. This cuts about 5s from my Debian kernel boot with the
> latest 6.0rc1 kernel (29s->24s).



Applied to target-arm.next, thanks.

-- PMM