[PATCH v2] xen/arm: do not read MVFR2 when is not defined

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

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@xxxxxxxxxx>
---
 xen/arch/arm/cpufeature.c           | 2 ++
 xen/include/asm-arm/arm32/sysregs.h | 3 +++
 2 files changed, 5 insertions(+)

diff --git a/xen/arch/arm/cpufeature.c b/xen/arch/arm/cpufeature.c
index 1f6a85aafe..698bfa0201 100644
--- 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 25cdcbfa4e..6841d5de43 100644
--- 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 */
-- 
2.17.1