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

[patchbot@xxxxxxx]

commit ce59e3dda5f99afbe7257e1e9a22dffd5c4d033c
Author:     Stefano Stabellini <sstabellini@xxxxxxxxxx>
AuthorDate: Tue Jan 5 11:05:48 2021 -0800
Commit:     Stefano Stabellini <sstabellini@xxxxxxxxxx>
CommitDate: Fri Jan 8 15:35:51 2021 -0800

    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@xxxxxxxxxx>
    Acked-by: Julien Grall <jgrall@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 */
--
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