KVM: SVM: Use kvm_is_cr4_bit_set() to query SMAP/SMEP in "can emulate"

Use kvm_is_cr4_bit_set() to query SMAP and SMEP when determining whether
or not AMD's SMAP+SEV errata prevents KVM from emulating an instruction.
This eliminates an implicit cast from ulong to bool and makes the code
slightly more readable.

Note, any overhead from making multiple calls to kvm_read_cr4_bits() is
negligible, not to mention the code is question is encountered only in
rare situations, i.e. is not a remotely hot path.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Binbin Wu <binbin.wu@linux.intel.com>
Link: https://lore.kernel.org/r/20230322045824.22970-4-binbin.wu@linux.intel.com
[sean: keep local smap/smep variables, massage changelog]
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: Binbin Wu

arch/x86/kvm/svm/svm.c

@@ -4545,7 +4545,6 @@ static bool svm_can_emulate_instruction(struct kvm_vcpu *vcpu, int emul_type,
 					void *insn, int insn_len)
 {
 	bool smep, smap, is_user;
-	unsigned long cr4;
 	u64 error_code;
 
 	/* Emulation is always possible when KVM has access to all guest state. */
@@ -4637,9 +4636,8 @@ static bool svm_can_emulate_instruction(struct kvm_vcpu *vcpu, int emul_type,
 	if (error_code & (PFERR_GUEST_PAGE_MASK | PFERR_FETCH_MASK))
 		goto resume_guest;
 
-	cr4 = kvm_read_cr4(vcpu);
-	smep = cr4 & X86_CR4_SMEP;
-	smap = cr4 & X86_CR4_SMAP;
+	smep = kvm_is_cr4_bit_set(vcpu, X86_CR4_SMEP);
+	smap = kvm_is_cr4_bit_set(vcpu, X86_CR4_SMAP);
 	is_user = svm_get_cpl(vcpu) == 3;
 	if (smap && (!smep || is_user)) {
 		pr_err_ratelimited("SEV Guest triggered AMD Erratum 1096\n");