KVM: arm64: Extract ESR_ELx.EC only

Since ARMv8.0 the upper 32 bits of ESR_ELx have been RES0, and recently
some of the upper bits gained a meaning and can be non-zero. For
example, when FEAT_LS64 is implemented, ESR_ELx[36:32] contain ISS2,
which for an ST64BV or ST64BV0 can be non-zero. This can be seen in ARM
DDI 0487G.b, page D13-3145, section D13.2.37.

Generally, we must not rely on RES0 bit remaining zero in future, and
when extracting ESR_ELx.EC we must mask out all other bits.

All C code uses the ESR_ELx_EC() macro, which masks out the irrelevant
bits, and therefore no alterations are required to C code to avoid
consuming irrelevant bits.

In a couple of places the KVM assembly extracts ESR_ELx.EC using LSR on
an X register, and so could in theory consume previously RES0 bits. In
both cases this is for comparison with EC values ESR_ELx_EC_HVC32 and
ESR_ELx_EC_HVC64, for which the upper bits of ESR_ELx must currently be
zero, but this could change in future.

This patch adjusts the KVM vectors to use UBFX rather than LSR to
extract ESR_ELx.EC, ensuring these are robust to future additions to
ESR_ELx.

Cc: [email protected]
Signed-off-by: Mark Rutland <[email protected]>
Cc: Alexandru Elisei <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: James Morse <[email protected]>
Cc: Marc Zyngier <[email protected]>
Cc: Suzuki K Poulose <[email protected]>
Cc: Will Deacon <[email protected]>
Acked-by: Will Deacon <[email protected]>
Signed-off-by: Marc Zyngier <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

arch/arm64/include/asm/esr.h

@@ -68,6 +68,7 @@
 #define ESR_ELx_EC_MAX		(0x3F)
 
 #define ESR_ELx_EC_SHIFT	(26)
+#define ESR_ELx_EC_WIDTH	(6)
 #define ESR_ELx_EC_MASK		(UL(0x3F) << ESR_ELx_EC_SHIFT)
 #define ESR_ELx_EC(esr)		(((esr) & ESR_ELx_EC_MASK) >> ESR_ELx_EC_SHIFT)
 

arch/arm64/kvm/hyp/hyp-entry.S

@@ -44,7 +44,7 @@
 el1_sync:				// Guest trapped into EL2
 
 	mrs	x0, esr_el2
-	lsr	x0, x0, #ESR_ELx_EC_SHIFT
+	ubfx	x0, x0, #ESR_ELx_EC_SHIFT, #ESR_ELx_EC_WIDTH
 	cmp	x0, #ESR_ELx_EC_HVC64
 	ccmp	x0, #ESR_ELx_EC_HVC32, #4, ne
 	b.ne	el1_trap

arch/arm64/kvm/hyp/nvhe/host.S

@@ -141,7 +141,7 @@ SYM_FUNC_END(__host_hvc)
 .L__vect_start\@:
 	stp	x0, x1, [sp, #-16]!
 	mrs	x0, esr_el2
-	lsr	x0, x0, #ESR_ELx_EC_SHIFT
+	ubfx	x0, x0, #ESR_ELx_EC_SHIFT, #ESR_ELx_EC_WIDTH
 	cmp	x0, #ESR_ELx_EC_HVC64
 	b.eq	__host_hvc
 	b	__host_exit