KVM: MMU: Inform users of mmio generation wraparound

From: Takuya Yoshikawa <[email protected]>

Without this information, users will just see unexpected performance problems and there is little chance we will get good reports from them:
note that mmio generation is increased even when we just start, or stop, dirty logging for some memory slot, in which case users cannot expect all shadow pages to be zapped.

printk_ratelimited() is used for this taking into account the problems that we can see the information many times when we start multiple VMs and guests can trigger this by reading ROM in a loop for example.

Signed-off-by Takuya Yoshikawa <[email protected]>

---
Interestingly, I saw this information printed twice every time. Looks like current_mmio_gen can become mmio_max_gen...

 arch/x86/kvm/mmu.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

arch/x86/kvm/mmu.c +0 -0

index c60c5da..54e3968 100644

arch/x86/kvm/mmu.c +2 -0

+		printk_ratelimited(KERN_INFO "kvm: zapping shadow pages for mmio generation wraparound\n");
 		kvm_mmu_invalidate_zap_all_pages(kvm);
+	}
 }
 
 static int mmu_shrink(struct shrinker *shrink, struct shrink_control *sc)

arch/x86/kvm/mmu.c void kvm_mmu_invalidate_mmio_sptes(struct kvm *kvm) · line 4388

Quoted 6 lines

arch/x86/kvm/mmu.c +1 -1

@@ -4385,8 +4385,10 @@ void kvm_mmu_invalidate_mmio_sptes(struct kvm *kvm)
 	 * The max value is MMIO_MAX_GEN - 1 since it is not called
 	 * when mark memslot invalid.
 	 */
-	if (unlikely(kvm_current_mmio_generation(kvm) >= (MMIO_MAX_GEN - 1)))
+	if (unlikely(kvm_current_mmio_generation(kvm) >= (MMIO_MAX_GEN - 1))) {

There is an off-by-one here (that I can fix in a separate patch). The right test is >= MMIO_MAX_GEN, since we have:

#define MMIO_MAX_GEN ((1 << MMIO_GEN_SHIFT) - 1)

Paolo

arch/x86/kvm/mmu.c void kvm_mmu_invalidate_mmio_sptes(struct kvm *kvm) · line 4392

Quoted 5 lines

arch/x86/kvm/mmu.c +2 -0

+		printk_ratelimited(KERN_INFO "kvm: zapping shadow pages for mmio generation wraparound\n");
 		kvm_mmu_invalidate_zap_all_pages(kvm);
+	}
 }