[PATCH 40/59] KVM: arm64: nv: Don't always start an S2 MMU search from the beginning

Marc Zyngier
2019-06-21T09:40:25Z

Starting a S2 MMU search from the beginning all the time means that
we're potentially nuking a useful context (like we'd potentially
have on a !VHE KVM guest).

Instead, let's always start the search from the point *after* the
last allocated context. This should ensure that alternating between
two EL1 contexts will not result in nuking the whole S2 each time.

lookup_s2_mmu now has a chance to provide a hit.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
---
arch/arm64/include/asm/kvm_host.h | 1 +
arch/arm64/kvm/nested.c | 14 ++++++++++++--
2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index b71a7a237f95..b7c44adcdbf3 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -123,6 +123,7 @@ struct kvm_arch {
*/
struct kvm_s2_mmu *nested_mmus;
size_t nested_mmus_size;
+ int nested_mmus_next;

/* VTCR_EL2 value for this VM */
u64 vtcr;
diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
index 09afafbdc8fe..214d59019935 100644
--- a/arch/arm64/kvm/nested.c
+++ b/arch/arm64/kvm/nested.c
@@ -363,14 +363,24 @@ static struct kvm_s2_mmu *get_s2_mmu_nested(struct kvm_vcpu *vcpu)
if (s2_mmu)
goto out;

- for (i = 0; i < kvm->arch.nested_mmus_size; i++) {
- s2_mmu = &kvm->arch.nested_mmus[i];
+ /*
+ * Make sure we don't always search from the same point, or we
+ * will always reuse a potentially active context, leaving
+ * free contexts unused.
+ */
+ for (i = kvm->arch.nested_mmus_next;
+ i < (kvm->arch.nested_mmus_size + kvm->arch.nested_mmus_next);
+ i++) {
+ s2_mmu = &kvm->arch.nested_mmus[i % kvm->arch.nested_mmus_size];

if (atomic_read(&s2_mmu->refcnt) == 0)
break;
}
BUG_ON(atomic_read(&s2_mmu->refcnt)); /* We have struct MMUs to spare */

+ /* Set the scene for the next search */
+ kvm->arch.nested_mmus_next = (i + 1) % kvm->arch.nested_mmus_size;
+
if (kvm_s2_mmu_valid(s2_mmu)) {
/* Clear the old state */
kvm_unmap_stage2_range(s2_mmu, 0, kvm_phys_size(kvm));
--
2.20.1