linux-kernel - [RFC PATCH v3 2/2] KVM: arm64: Distinguish cases of memcache allocations completely

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Message-ID: <20210326031654.3716-3-wangyanan55@huawei.com>
Date:   Fri, 26 Mar 2021 11:16:54 +0800
From:   Yanan Wang <wangyanan55@...wei.com>
To:     Marc Zyngier <maz@...nel.org>, Will Deacon <will@...nel.org>,
        "Alexandru Elisei" <alexandru.elisei@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        <kvmarm@...ts.cs.columbia.edu>,
        <linux-arm-kernel@...ts.infradead.org>, <kvm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
CC:     James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Gavin Shan <gshan@...hat.com>,
        Quentin Perret <qperret@...gle.com>,
        <wanghaibin.wang@...wei.com>, <zhukeqian1@...wei.com>,
        <yuzenghui@...wei.com>, Yanan Wang <wangyanan55@...wei.com>
Subject: [RFC PATCH v3 2/2] KVM: arm64: Distinguish cases of memcache allocations completely

With a guest translation fault, the memcache pages are not needed if KVM
is only about to install a new leaf entry into the existing page table.
And with a guest permission fault, the memcache pages are also not needed
for a write_fault in dirty-logging time if KVM is only about to update
the existing leaf entry instead of collapsing a block entry into a table.

By comparing fault_granule and vma_pagesize, cases that require allocations
from memcache and cases that don't can be distinguished completely.

Signed-off-by: Yanan Wang <wangyanan55@...wei.com>
---
 arch/arm64/kvm/mmu.c | 25 ++++++++++++-------------
 1 file changed, 12 insertions(+), 13 deletions(-)

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 1eec9f63bc6f..05af40dc60c1 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -810,19 +810,6 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
 	gfn = fault_ipa >> PAGE_SHIFT;
 	mmap_read_unlock(current->mm);
 
-	/*
-	 * Permission faults just need to update the existing leaf entry,
-	 * and so normally don't require allocations from the memcache. The
-	 * only exception to this is when dirty logging is enabled at runtime
-	 * and a write fault needs to collapse a block entry into a table.
-	 */
-	if (fault_status != FSC_PERM || (logging_active && write_fault)) {
-		ret = kvm_mmu_topup_memory_cache(memcache,
-						 kvm_mmu_cache_min_pages(kvm));
-		if (ret)
-			return ret;
-	}
-
 	mmu_seq = vcpu->kvm->mmu_notifier_seq;
 	/*
 	 * Ensure the read of mmu_notifier_seq happens before we call
@@ -880,6 +867,18 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
 	else if (cpus_have_const_cap(ARM64_HAS_CACHE_DIC))
 		prot |= KVM_PGTABLE_PROT_X;
 
+	/*
+	 * Allocations from the memcache are required only when granule of the
+	 * lookup level where the guest fault happened exceeds vma_pagesize,
+	 * which means new page tables will be created in the fault handlers.
+	 */
+	if (fault_granule > vma_pagesize) {
+		ret = kvm_mmu_topup_memory_cache(memcache,
+						 kvm_mmu_cache_min_pages(kvm));
+		if (ret)
+			return ret;
+	}
+
 	/*
 	 * Under the premise of getting a FSC_PERM fault, we just need to relax
 	 * permissions only if vma_pagesize equals fault_granule. Otherwise,
-- 
2.19.1