|author||Sean Christopherson <email@example.com>||2019-02-05 13:01:17 -0800|
|committer||Paolo Bonzini <firstname.lastname@example.org>||2019-02-20 22:48:36 +0100|
KVM: Remove the hack to trigger memslot generation wraparound
x86 captures a subset of the memslot generation (19 bits) in its MMIO sptes so that it can expedite emulated MMIO handling by checking only the releveant spte, i.e. doesn't need to do a full page fault walk. Because the MMIO sptes capture only 19 bits (due to limited space in the sptes), there is a non-zero probability that the MMIO generation could wrap, e.g. after 500k memslot updates. Since normal usage is extremely unlikely to result in 500k memslot updates, a hack was added by commit 69c9ea93eaea ("KVM: MMU: init kvm generation close to mmio wrap-around value") to offset the MMIO generation in order to trigger a wraparound, e.g. after 150 memslot updates. When separate memslot generation sequences were assigned to each address space, commit 00f034a12fdd ("KVM: do not bias the generation number in kvm_current_mmio_generation") moved the offset logic into the initialization of the memslot generation itself so that the per-address space bit(s) were not dropped/corrupted by the MMIO shenanigans. Remove the offset hack for three reasons: - While it does exercise x86's kvm_mmu_invalidate_mmio_sptes(), simply wrapping the generation doesn't actually test the interesting case of having stale MMIO sptes with the new generation number, e.g. old sptes with a generation number of 0. - Triggering kvm_mmu_invalidate_mmio_sptes() prematurely makes its performance rather important since the probability of invalidating MMIO sptes jumps from "effectively never" to "fairly likely". This limits what can be done in future patches, e.g. to simplify the invalidation code, as doing so without proper caution could lead to a noticeable performance regression. - Forcing the memslots generation, which is a 64-bit number, to wrap prevents KVM from assuming the memslots generation will never wrap. This in turn prevents KVM from using an arbitrary bit for the "update in-progress" flag, e.g. using bit 63 would immediately collide with using a large value as the starting generation number. The "update in-progress" flag is effectively forced into bit 0 so that it's (subtly) taken into account when incrementing the generation. Signed-off-by: Sean Christopherson <email@example.com> Signed-off-by: Paolo Bonzini <firstname.lastname@example.org>
1 files changed, 2 insertions, 6 deletions
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 0f1f1c7c7a36..5c2e7e173a46 100644
@@ -656,12 +656,8 @@ static struct kvm *kvm_create_vm(unsigned long type)
struct kvm_memslots *slots = kvm_alloc_memslots();
- * Generations must be different for each address space.
- * Init kvm generation close to the maximum to easily test the
- * code of handling generation number wrap-around.
- slots->generation = i * 2 - 150;
+ /* Generations must be different for each address space. */
+ slots->generation = i * 2;