[PATCH V3 03/12] KVM: X86/MMU: Reduce a check in using_local_root_page() for common cases

Lai Jiangshan
2022-05-21T13:16:40Z

From: Lai Jiangshan <jiangshan.ljs@antgroup.com>

For most cases, mmu->root_role.direct is true and mmu->root_role.level
is not PT32E_ROOT_LEVEL which means using_local_root_page() is often
checking for all the three test which is not good in fast paths.

Morph the conditions in using_local_root_page() to an equivalent one
to reduce a check.

Signed-off-by: Lai Jiangshan <jiangshan.ljs@antgroup.com>
---
arch/x86/kvm/mmu/mmu.c | 45 ++++++++++++++++++++++++++++++++++++++++--
1 file changed, 43 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 624b6d2473f7..240ebe589caf 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -1716,11 +1716,52 @@ static void drop_parent_pte(struct kvm_mmu_page *sp,
*
* (There is no "mmu->root_role.level > PT32E_ROOT_LEVEL" here, because it is
* already ensured that mmu->root_role.level >= PT32E_ROOT_LEVEL)
+ *
+ * But mmu->root_role.direct is normally true and mmu->root_role.level is
+ * normally not PT32E_ROOT_LEVEL. To reduce a check for the fast path of
+ * fast_pgd_switch() in mormal case, mmu->root_role.direct is checked first.
+ *
+ * The return value is:
+ * mmu->root_role.level == PT32E_ROOT_LEVEL ||
+ * (!mmu->root_role.direct && mmu->cpu_role.base.level <= PT32E_ROOT_LEVEL)
+ * =>
+ * (mmu->root_role.direct && mmu->root_role.level == PT32E_ROOT_LEVEL) ||
+ * (!mmu->root_role.direct && mmu->root_role.level == PT32E_ROOT_LEVEL) ||
+ * (!mmu->root_role.direct && mmu->cpu_role.base.level <= PT32E_ROOT_LEVEL)
+ * =>
+ * (mmu->root_role.direct && mmu->root_role.level == PT32E_ROOT_LEVEL) ||
+ * (!mmu->root_role.direct &&
+ * (mmu->root_role.level == PT32E_ROOT_LEVEL ||
+ * mmu->cpu_role.base.level <= PT32E_ROOT_LEVEL))
+ * => (for !direct, mmu->root_role.level == PT32E_ROOT_LEVEL implies
+ * mmu->cpu_role.base.level <= PT32E_ROOT_LEVEL)
+ * =>
+ * (mmu->root_role.direct && mmu->root_role.level == PT32E_ROOT_LEVEL) ||
+ * (!mmu->root_role.direct && mmu->cpu_role.base.level <= PT32E_ROOT_LEVEL)
+ *
+ * In other words:
+ *
+ * For the first and third cases, it is
+ * mmu->root_role.direct && mmu->root_role.level == PT32E_ROOT_LEVEL
+ * And if this condition is true, it must be one of the two cases.
+ *
+ * For the 2nd, 4th and 5th cases, it is
+ * !mmu->root_role.direct && mmu->cpu_role.base.level <= PT32E_ROOT_LEVEL
+ * And if this condition is true, it must be one of the three cases although
+ * it is not so intuitive. It can be split into:
+ * mmu->root_role.level == PT32E_ROOT_LEVEL &&
+ * (!mmu->root_role.direct && mmu->cpu_role.base.level <= PT32E_ROOT_LEVEL)
+ * which is for the 2nd and 4th cases and
+ * mmu->root_role.level > PT32E_ROOT_LEVEL &&
+ * !mmu->root_role.direct && mmu->cpu_role.base.level <= PT32E_ROOT_LEVEL
+ * which is the last case.
*/
static bool using_local_root_page(struct kvm_mmu *mmu)
{
- return mmu->root_role.level == PT32E_ROOT_LEVEL ||
- (!mmu->root_role.direct && mmu->cpu_role.base.level <= PT32E_ROOT_LEVEL);
+ if (mmu->root_role.direct)
+ return mmu->root_role.level == PT32E_ROOT_LEVEL;
+ else
+ return mmu->cpu_role.base.level <= PT32E_ROOT_LEVEL;
}

static struct kvm_mmu_page *kvm_mmu_alloc_page(struct kvm_vcpu *vcpu, int direct)
--
2.19.1.6.gb485710b