xen/arm: do not read MVFR2 when is not defined

MVFR2 is not available on ARMv7. It is available on ARMv8 aarch32 and
aarch64. If Xen reads MVFR2 on ARMv7 it could crash.

Avoid the issue by doing the following:

- define MVFR2_MAYBE_UNDEFINED on arm32
- if MVFR2_MAYBE_UNDEFINED, do not attempt to read MVFR2 in Xen
- keep the 3rd register_t in struct cpuinfo_arm.mvfr on arm32 so that a
  guest read to the register returns '0' instead of crashing the guest.

'0' is an appropriate value to return to the guest because it is defined
as "no support for miscellaneous features".

Aarch64 Xen is not affected by this patch.

Fixes: 9cfdb489af81 ("xen/arm: Add ID registers and complete cpuinfo")
Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Acked-by: Julien Grall <jgrall@amazon.com>

diff --git a/xen/arch/arm/cpufeature.c b/xen/arch/arm/cpufeature.c
index 1f6a85aafe2a7063cc7014a9072ea1bedc553e60..698bfa02013925f17d3f2c090b4d8dd77fc198ec 100644 (file)
--- a/xen/arch/arm/cpufeature.c
+++ b/xen/arch/arm/cpufeature.c
@@ -150,7 +150,9 @@ void identify_cpu(struct cpuinfo_arm *c)
 
         c->mvfr.bits[0] = READ_SYSREG(MVFR0_EL1);
         c->mvfr.bits[1] = READ_SYSREG(MVFR1_EL1);
+#ifndef MVFR2_MAYBE_UNDEFINED
         c->mvfr.bits[2] = READ_SYSREG(MVFR2_EL1);
+#endif
 }
 
 /*

diff --git a/xen/include/asm-arm/arm32/sysregs.h b/xen/include/asm-arm/arm32/sysregs.h
index 25cdcbfa4ecf634c434c458045d40d39e5136c83..6841d5de43bf2cb6ba04357d003a124571ffcd56 100644 (file)
--- a/xen/include/asm-arm/arm32/sysregs.h
+++ b/xen/include/asm-arm/arm32/sysregs.h
@@ -62,6 +62,9 @@
 #define READ_SYSREG(R...)       READ_SYSREG32(R)
 #define WRITE_SYSREG(V, R...)   WRITE_SYSREG32(V, R)
 
+/* MVFR2 is not defined on ARMv7 */
+#define MVFR2_MAYBE_UNDEFINED
+
 #endif /* __ASSEMBLY__ */
 
 #endif /* __ASM_ARM_ARM32_SYSREGS_H */