KVM: arm64: Skip more of the SError vaxorcism

During __guest_exit() we need to consume any SError left pending by the
guest so it doesn't contaminate the host. With v8.2 we use the
ESB-instruction. For systems without v8.2, we use dsb+isb and unmask
SError. We do this on every guest exit.

Use the same dsb+isr_el1 trick, this lets us know if an SError is pending
after the dsb, allowing us to skip the isb and self-synchronising PSTATE
write if its not.

This means SError remains masked during KVM's world-switch, so any SError
that occurs during this time is reported by the host, instead of causing
a hyp-panic.

As we're benchmarking this code lets polish the layout. If you give gcc
likely()/unlikely() hints in an if() condition, it shuffles the generated
assembly so that the likely case is immediately after the branch. Lets
do the same here.
Signed-off-by: James Morse <james.morse@arm.com>

Changes since v2:
 * Added isb after the dsb to prevent an early read
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

arch/arm64/kvm/hyp/entry.S

@@ -151,8 +151,16 @@ alternative_if ARM64_HAS_RAS_EXTN
 	orr	x0, x0, #(1<<ARM_EXIT_WITH_SERROR_BIT)
 1:	ret
 alternative_else
-	// If we have a pending asynchronous abort, now is the
-	// time to find out. From your VAXorcist book, page 666:
+	dsb	sy		// Synchronize against in-flight ld/st
+	isb			// Prevent an early read of side-effect free ISR
+	mrs	x2, isr_el1
+	tbnz	x2, #8, 2f	// ISR_EL1.A
+	ret
+	nop
+2:
+alternative_endif
+	// We know we have a pending asynchronous abort, now is the
+	// time to flush it out. From your VAXorcist book, page 666:
 	// "Threaten me not, oh Evil one!  For I speak with
 	// the power of DEC, and I command thee to show thyself!"
 	mrs	x2, elr_el2
@@ -160,9 +168,7 @@ alternative_else
 	mrs	x4, spsr_el2
 	mov	x5, x0
 
-	dsb	sy		// Synchronize against in-flight ld/st
 	msr	daifclr, #4	// Unmask aborts
-alternative_endif
 
 	// This is our single instruction exception window. A pending
 	// SError is guaranteed to occur at the earliest when we unmask