x86/speculation: Use ARCH_CAPABILITIES to skip L1D flush on vmentry

Bit 3 of ARCH_CAPABILITIES tells a hypervisor that L1D flush on vmentry is
not needed.  Add a new value to enum vmx_l1d_flush_state, which is used
either if there is no L1TF bug at all, or if bit 3 is set in ARCH_CAPABILITIES.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

CVE-2018-3620
CVE-2018-3646
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

arch/x86/include/asm/msr-index.h

@@ -58,6 +58,7 @@
 #define MSR_IA32_ARCH_CAPABILITIES	0x0000010a
 #define ARCH_CAP_RDCL_NO		(1 << 0)   /* Not susceptible to Meltdown */
 #define ARCH_CAP_IBRS_ALL		(1 << 1)   /* Enhanced IBRS support */
+#define ARCH_CAP_SKIP_VMENTRY_L1DFLUSH	(1 << 3)   /* Skip L1D flush on vmentry */
 #define ARCH_CAP_SSB_NO			(1 << 4)   /*
 						    * Not susceptible to Speculative Store Bypass
 						    * attack, so no Speculative Store Bypass

arch/x86/include/asm/vmx.h

@@ -506,6 +506,7 @@ enum vmx_l1d_flush_state {
 	VMENTER_L1D_FLUSH_COND,
 	VMENTER_L1D_FLUSH_ALWAYS,
 	VMENTER_L1D_FLUSH_EPT_DISABLED,
+	VMENTER_L1D_FLUSH_NOT_REQUIRED,
 };
 
 extern enum vmx_l1d_flush_state l1tf_vmx_mitigation;

arch/x86/kernel/cpu/bugs.c

@@ -780,6 +780,7 @@ static const char *l1tf_vmx_states[] = {
 	[VMENTER_L1D_FLUSH_COND]		= "conditional cache flushes",
 	[VMENTER_L1D_FLUSH_ALWAYS]		= "cache flushes",
 	[VMENTER_L1D_FLUSH_EPT_DISABLED]	= "EPT disabled",
+	[VMENTER_L1D_FLUSH_NOT_REQUIRED]	= "flush not necessary"
 };
 
 static ssize_t l1tf_show_state(char *buf)

arch/x86/kvm/vmx.c

@@ -203,6 +203,16 @@ static int vmx_setup_l1d_flush(enum vmx_l1d_flush_state l1tf)
 		return 0;
 	}
 
+       if (boot_cpu_has(X86_FEATURE_ARCH_CAPABILITIES)) {
+	       u64 msr;
+
+	       rdmsrl(MSR_IA32_ARCH_CAPABILITIES, msr);
+	       if (msr & ARCH_CAP_SKIP_VMENTRY_L1DFLUSH) {
+		       l1tf_vmx_mitigation = VMENTER_L1D_FLUSH_NOT_REQUIRED;
+		       return 0;
+	       }
+       }
+
 	/* If set to auto use the default l1tf mitigation method */
 	if (l1tf == VMENTER_L1D_FLUSH_AUTO) {
 		switch (l1tf_mitigation) {