KVM: x86: Disallow guest CPUID lookups when IRQs are disabled

Now that KVM has a framework for caching guest CPUID feature flags, add
a "rule" that IRQs must be enabled when doing guest CPUID lookups, and
enforce the rule via a lockdep assertion.  CPUID lookups are slow, and
within KVM, IRQs are only ever disabled in hot paths, e.g. the core run
loop, fast page fault handling, etc.  I.e. querying guest CPUID with IRQs
disabled, especially in the run loop, should be avoided.

Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Sean Christopherson <[email protected]>

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 67e9f79fe059758b71936dd195680e13b38278e4..e961e9a058477c4b66991a129128748073a97bcf 100644 (file)
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -11,6 +11,7 @@
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 
 #include <linux/kvm_host.h>
+#include "linux/lockdep.h"
 #include <linux/export.h>
 #include <linux/vmalloc.h>
 #include <linux/uaccess.h>
@@ -84,6 +85,18 @@ static inline struct kvm_cpuid_entry2 *cpuid_entry2_find(
        struct kvm_cpuid_entry2 *e;
        int i;
 
+       /*
+        * KVM has a semi-arbitrary rule that querying the guest's CPUID model
+        * with IRQs disabled is disallowed.  The CPUID model can legitimately
+        * have over one hundred entries, i.e. the lookup is slow, and IRQs are
+        * typically disabled in KVM only when KVM is in a performance critical
+        * path, e.g. the core VM-Enter/VM-Exit run loop.  Nothing will break
+        * if this rule is violated, this assertion is purely to flag potential
+        * performance issues.  If this fires, consider moving the lookup out
+        * of the hotpath, e.g. by caching information during CPUID updates.
+        */
+       lockdep_assert_irqs_enabled();
+
        for (i = 0; i < nent; i++) {
                e = &entries[i];