[BOOT-WRAPPER PATCH 2/3] aarch64: Enable use of FPMR for EL2 and below

[Mark Rutland]

FEAT_FPMR adds the FPMR register. Acceses to FPMR (whether direct or
indirect) trap to EL3 unless SCR_EL3.EnFPM is set, and so boot-wrapper
support is necessary.

Support for FEAT_FPMR was added to Linux in v6.8 without any
boot-wrapper support. Consequently when FPMR is enabled in a model, the
kernel will hang when attempting to write to the FPMR (e.g. when
entering userspace for the first time).

Add boot-wrapper support for FEAT_FPMR. As FPMR is not described in the
latest ARM ARM (ARM DDI 0487K.a), the relevant definitions are taken
from the 2024-09 release of the "Arm Architecture Registers" document,
ARM DDI 0601 (ID092424), which can be found at:

  https://developer.arm.com/documentation/ddi0601/2024-09/?lang=en

The ID_AA64PFR2_EL1 ID register has existed as reserved RES0 space since
ARMv8.0 but only recently gained a name, and so older assemblers may not
be able to encode ID_AA64PFR2_EL1 directly. Thus we need an explicit
definition of the sysreg encoding to support these assemblers.

Signed-off-by: Mark Rutland <mark.rutland at arm.com>
Cc: Mark Brown <broonie at kernel.org>
---
 arch/aarch64/include/asm/cpu.h | 4 ++++
 arch/aarch64/init.c            | 3 +++
 2 files changed, 7 insertions(+)

diff --git a/arch/aarch64/include/asm/cpu.h b/arch/aarch64/include/asm/cpu.h
index 6fa11da..3ef58f3 100644
--- a/arch/aarch64/include/asm/cpu.h
+++ b/arch/aarch64/include/asm/cpu.h
@@ -68,6 +68,7 @@
 #define SCR_EL3_SCTLR2En		BIT(44)
 #define SCR_EL3_PIEN			BIT(45)
 #define SCR_EL3_D128En			BIT(47)
+#define SCR_EL3_EnFPM			BIT(50)
 
 #define VTCR_EL2_MSA			BIT(31)
 
@@ -116,6 +117,9 @@
 #define ID_AA64PFR1_EL1_CSV2_frac	BITS(35, 32)
 #define ID_AA64PFR1_EL1_THE		BITS(51, 48)
 
+#define ID_AA64PFR2_EL1			s3_0_c0_c4_2
+#define ID_AA64PFR2_EL1_FPMR		BITS(35, 32)
+
 #define ID_AA64SMFR0_EL1		s3_0_c0_c4_5
 #define ID_AA64SMFR0_EL1_FA64		BIT(63)
 
diff --git a/arch/aarch64/init.c b/arch/aarch64/init.c
index da036bb..1f38516 100644
--- a/arch/aarch64/init.c
+++ b/arch/aarch64/init.c
@@ -124,6 +124,9 @@ static void cpu_init_el3(void)
 	if (mrs_field(ID_AA64PFR1_EL1, THE))
 		scr |= SCR_EL3_RCWMASKEn;
 
+	if (mrs_field(ID_AA64PFR2_EL1, FPMR))
+		scr |= SCR_EL3_EnFPM;
+
 	msr(SCR_EL3, scr);
 
 	msr(CPTR_EL3, cptr);
-- 
2.30.2