[Qemu-devel] [PATCH v2 32/32] target/arm: generate a custom MIDR for -cpu max

[Richard Henderson <richard.henderson@linaro.org>]

From: Alex Bennée <alex.bennee@linaro.org>

While most features are now detected by probing the ID_* registers
kernels can (and do) use MIDR_EL1 for working out of they have to
apply errata. This can trip up warnings in the kernel as it tries to
work out if it should apply workarounds to features that don't
actually exist in the reported CPU type.

Avoid this problem by synthesising our own MIDR value.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20190726113950.7499-1-alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
---
 target/arm/cpu.h   |  6 ++++++
 target/arm/cpu64.c | 19 +++++++++++++++++++
 2 files changed, 25 insertions(+)

diff --git a/target/arm/cpu.h b/target/arm/cpu.h
index c41da1d791..bc9b6db9b4 100644
--- a/target/arm/cpu.h
+++ b/target/arm/cpu.h
@@ -1605,6 +1605,12 @@ FIELD(V7M_FPCCR, ASPEN, 31, 1)
 /*
  * System register ID fields.
  */
+FIELD(MIDR_EL1, REVISION, 0, 4)
+FIELD(MIDR_EL1, PARTNUM, 4, 12)
+FIELD(MIDR_EL1, ARCHITECTURE, 16, 4)
+FIELD(MIDR_EL1, VARIANT, 20, 4)
+FIELD(MIDR_EL1, IMPLEMENTER, 24, 8)
+
 FIELD(ID_ISAR0, SWAP, 0, 4)
 FIELD(ID_ISAR0, BITCOUNT, 4, 4)
 FIELD(ID_ISAR0, BITFIELD, 8, 4)
diff --git a/target/arm/cpu64.c b/target/arm/cpu64.c
index b1bb394c6d..3a1e98a18e 100644
--- a/target/arm/cpu64.c
+++ b/target/arm/cpu64.c
@@ -296,6 +296,25 @@ static void aarch64_max_initfn(Object *obj)
         uint32_t u;
         aarch64_a57_initfn(obj);
 
+        /*
+         * Reset MIDR so the guest doesn't mistake our 'max' CPU type for a real
+         * one and try to apply errata workarounds or use impdef features we
+         * don't provide.
+         * An IMPLEMENTER field of 0 means "reserved for software use";
+         * ARCHITECTURE must be 0xf indicating "v7 or later, check ID registers
+         * to see which features are present";
+         * the VARIANT, PARTNUM and REVISION fields are all implementation
+         * defined and we choose to define PARTNUM just in case guest
+         * code needs to distinguish this QEMU CPU from other software
+         * implementations, though this shouldn't be needed.
+         */
+        t = FIELD_DP64(0, MIDR_EL1, IMPLEMENTER, 0);
+        t = FIELD_DP64(t, MIDR_EL1, ARCHITECTURE, 0xf);
+        t = FIELD_DP64(t, MIDR_EL1, PARTNUM, 'Q');
+        t = FIELD_DP64(t, MIDR_EL1, VARIANT, 0);
+        t = FIELD_DP64(t, MIDR_EL1, REVISION, 0);
+        cpu->midr = t;
+
         t = cpu->isar.id_aa64isar0;
         t = FIELD_DP64(t, ID_AA64ISAR0, AES, 2); /* AES + PMULL */
         t = FIELD_DP64(t, ID_AA64ISAR0, SHA1, 1);
-- 
2.17.1