x86/cpu: Add a VMX flag to enumerate 5-level EPT support to userspace

Add a VMX flag in /proc/cpuinfo, ept_5level, so that userspace can query
whether or not the CPU supports 5-level EPT paging.  EPT capabilities are
enumerated via MSR, i.e. aren't accessible to userspace without help from
the kernel, and knowing whether or not 5-level EPT is supported is useful
for debug, triage, testing, etc.

For example, when EPT is enabled, bits 51:48 of guest physical addresses
are consumed by the CPU if and only if 5-level EPT is enabled.  For CPUs
with MAXPHYADDR > 48, KVM *can't* map all legal guest memory without
5-level EPT, making 5-level EPT support valuable information for userspace.

Reported-by: Yi Lai <yi1.lai@intel.com>
Cc: Tao Su <tao1.su@linux.intel.com>
Cc: Xudong Hao <xudong.hao@intel.com>
Link: https://lore.kernel.org/r/20240110002340.485595-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/include/asm/vmxfeatures.h

@@ -25,6 +25,7 @@
 #define VMX_FEATURE_EPT_EXECUTE_ONLY	( 0*32+ 17) /* "ept_x_only" EPT entries can be execute only */
 #define VMX_FEATURE_EPT_AD		( 0*32+ 18) /* EPT Accessed/Dirty bits */
 #define VMX_FEATURE_EPT_1GB		( 0*32+ 19) /* 1GB EPT pages */
+#define VMX_FEATURE_EPT_5LEVEL		( 0*32+ 20) /* 5-level EPT paging */
 
 /* Aggregated APIC features 24-27 */
 #define VMX_FEATURE_FLEXPRIORITY	( 0*32+ 24) /* TPR shadow + virt APIC */

arch/x86/kernel/cpu/feat_ctl.c

@@ -72,6 +72,8 @@ static void init_vmx_capabilities(struct cpuinfo_x86 *c)
 		c->vmx_capability[MISC_FEATURES] |= VMX_F(EPT_AD);
 	if (ept & VMX_EPT_1GB_PAGE_BIT)
 		c->vmx_capability[MISC_FEATURES] |= VMX_F(EPT_1GB);
+	if (ept & VMX_EPT_PAGE_WALK_5_BIT)
+		c->vmx_capability[MISC_FEATURES] |= VMX_F(EPT_5LEVEL);
 
 	/* Synthetic APIC features that are aggregates of multiple features. */
 	if ((c->vmx_capability[PRIMARY_CTLS] & VMX_F(VIRTUAL_TPR)) &&