[PATCH v2 2/3] x86: Refactor conditional guard in probe_cpuid_faulting()

[Alejandro Vallejo <alejandro.vallejo@cloud.com>]

Move vendor-specific checks to the vendor-specific callers.

No functional change.

Signed-off-by: Alejandro Vallejo <alejandro.vallejo@cloud.com>
---
v2:
  * Patch factored out from patch2 of v1
---
 xen/arch/x86/cpu/amd.c    | 10 +++++++++-
 xen/arch/x86/cpu/common.c | 11 -----------
 xen/arch/x86/cpu/intel.c  |  9 ++++++++-
 3 files changed, 17 insertions(+), 13 deletions(-)

diff --git a/xen/arch/x86/cpu/amd.c b/xen/arch/x86/cpu/amd.c
index caafe44740..899bae7a10 100644
--- a/xen/arch/x86/cpu/amd.c
+++ b/xen/arch/x86/cpu/amd.c
@@ -271,7 +271,15 @@ static void __init noinline amd_init_levelling(void)
 {
 	const struct cpuidmask *m = NULL;
 
-	if (probe_cpuid_faulting())
+	/*
+	 * If there's support for CpuidUserDis or CPUID faulting then
+	 * we can skip levelling because CPUID accesses are trapped anyway.
+	 *
+	 * CPUID faulting is an Intel feature analogous to CpuidUserDis, so
+	 * that can only be present when Xen is itself virtualized (because
+	 * it can be emulated)
+	 */
+	if (cpu_has_hypervisor && probe_cpuid_faulting())
 		return;
 
 	probe_masking_msrs();
diff --git a/xen/arch/x86/cpu/common.c b/xen/arch/x86/cpu/common.c
index edc4db1335..4bfaac4590 100644
--- a/xen/arch/x86/cpu/common.c
+++ b/xen/arch/x86/cpu/common.c
@@ -131,17 +131,6 @@ bool __init probe_cpuid_faulting(void)
 	uint64_t val;
 	int rc;
 
-	/*
-	 * Don't bother looking for CPUID faulting if we aren't virtualised on
-	 * AMD or Hygon hardware - it won't be present.  Likewise for Fam0F
-	 * Intel hardware.
-	 */
-	if (((boot_cpu_data.x86_vendor & (X86_VENDOR_AMD | X86_VENDOR_HYGON)) ||
-	     ((boot_cpu_data.x86_vendor == X86_VENDOR_INTEL) &&
-	      boot_cpu_data.x86 == 0xf)) &&
-	    !cpu_has_hypervisor)
-		return false;
-
 	if ((rc = rdmsr_safe(MSR_INTEL_PLATFORM_INFO, val)) == 0)
 		raw_cpu_policy.platform_info.cpuid_faulting =
 			val & MSR_PLATFORM_INFO_CPUID_FAULTING;
diff --git a/xen/arch/x86/cpu/intel.c b/xen/arch/x86/cpu/intel.c
index 71fc1a1e18..a04414ba1d 100644
--- a/xen/arch/x86/cpu/intel.c
+++ b/xen/arch/x86/cpu/intel.c
@@ -226,7 +226,14 @@ static void cf_check intel_ctxt_switch_masking(const struct vcpu *next)
  */
 static void __init noinline intel_init_levelling(void)
 {
-	if (probe_cpuid_faulting())
+	/*
+	 * Intel Fam0f is old enough that probing for CPUID faulting support
+	 * introduces spurious #GP(0) when the appropriate MSRs are read,
+	 * so skip it altogether. In the case where Xen is virtualized these
+	 * MSRs may be emulated though, so we allow it in that case.
+	 */
+	if ((boot_cpu_data.x86 != 0xf || cpu_has_hypervisor) &&
+	    probe_cpuid_faulting())
 		return;
 
 	probe_masking_msrs();
-- 
2.34.1