kvm: arm64: Convert kvm_set_s2pte_readonly() from inline asm to cmpxchg()

To take advantage of the LSE atomic instructions and also make the code
cleaner, convert the kvm_set_s2pte_readonly() function to use the more
generic cmpxchg().

Cc: Marc Zyngier <marc.zyngier@arm.com>
Reviewed-by: Will Deacon <will.deacon@arm.com>
Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/kvm_mmu.h

@@ -175,18 +175,15 @@ static inline pmd_t kvm_s2pmd_mkwrite(pmd_t pmd)
 
 static inline void kvm_set_s2pte_readonly(pte_t *pte)
 {
-	pteval_t pteval;
-	unsigned long tmp;
-
-	asm volatile("//	kvm_set_s2pte_readonly\n"
-	"	prfm	pstl1strm, %2\n"
-	"1:	ldxr	%0, %2\n"
-	"	and	%0, %0, %3		// clear PTE_S2_RDWR\n"
-	"	orr	%0, %0, %4		// set PTE_S2_RDONLY\n"
-	"	stxr	%w1, %0, %2\n"
-	"	cbnz	%w1, 1b\n"
-	: "=&r" (pteval), "=&r" (tmp), "+Q" (pte_val(*pte))
-	: "L" (~PTE_S2_RDWR), "L" (PTE_S2_RDONLY));
+	pteval_t old_pteval, pteval;
+
+	pteval = READ_ONCE(pte_val(*pte));
+	do {
+		old_pteval = pteval;
+		pteval &= ~PTE_S2_RDWR;
+		pteval |= PTE_S2_RDONLY;
+		pteval = cmpxchg_relaxed(&pte_val(*pte), old_pteval, pteval);
+	} while (pteval != old_pteval);
 }
 
 static inline bool kvm_s2pte_readonly(pte_t *pte)