KVM: x86: Remove manual pfn lookup when retrying #PF after failed emulation

Drop the manual pfn look when retrying an instruction that KVM failed to
emulation in response to a #PF due to a write-protected gfn.  Now that KVM
sets EMULTYPE_ALLOW_RETRY_PF if and only if the page fault hit a write-
protected gfn, i.e. if and only if there's a writable memslot, there's no
need to redo the lookup to avoid retrying an instruction that failed on
emulated MMIO (no slot, or a write to a read-only slot).

I.e. KVM will never attempt to retry an instruction that failed on
emulated MMIO, whereas that was not the case prior to the introduction of
RET_PF_WRITE_PROTECTED.
Reviewed-by: Yuan Yao <yuan.yao@intel.com>
Link: https://lore.kernel.org/r/20240831001538.336683-16-seanjc@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/x86.c

@@ -8865,7 +8865,6 @@ static bool reexecute_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
 				  int emulation_type)
 {
 	gpa_t gpa = cr2_or_gpa;
-	kvm_pfn_t pfn;
 
 	if (!(emulation_type & EMULTYPE_ALLOW_RETRY_PF))
 		return false;
@@ -8885,23 +8884,6 @@ static bool reexecute_instruction(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
 			return true;
 	}
 
-	/*
-	 * Do not retry the unhandleable instruction if it faults on the
-	 * readonly host memory, otherwise it will goto a infinite loop:
-	 * retry instruction -> write #PF -> emulation fail -> retry
-	 * instruction -> ...
-	 */
-	pfn = gfn_to_pfn(vcpu->kvm, gpa_to_gfn(gpa));
-
-	/*
-	 * If the instruction failed on the error pfn, it can not be fixed,
-	 * report the error to userspace.
-	 */
-	if (is_error_noslot_pfn(pfn))
-		return false;
-
-	kvm_release_pfn_clean(pfn);
-
 	/*
 	 * If emulation may have been triggered by a write to a shadowed page
 	 * table, unprotect the gfn (zap any relevant SPTEs) and re-enter the