linux-kernel - [RFC PATCH 11/18] KVM: arm64: Add vCPU memory fault fast-path for Userfault

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Message-ID: <20240710234222.2333120-12-jthoughton@google.com>
Date: Wed, 10 Jul 2024 23:42:15 +0000
From: James Houghton <jthoughton@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: Marc Zyngier <maz@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>, 
	James Morse <james.morse@....com>, Suzuki K Poulose <suzuki.poulose@....com>, 
	Zenghui Yu <yuzenghui@...wei.com>, Sean Christopherson <seanjc@...gle.com>, Shuah Khan <shuah@...nel.org>, 
	Peter Xu <peterx@...hat.org>, Axel Rasmussen <axelrasmussen@...gle.com>, 
	David Matlack <dmatlack@...gle.com>, James Houghton <jthoughton@...gle.com>, kvm@...r.kernel.org, 
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev
Subject: [RFC PATCH 11/18] KVM: arm64: Add vCPU memory fault fast-path for Userfault

Make this optimization for the same reason we make it for x86: because
it necessary for sufficient post-copy performance when scaling up to
hundreds of cores (even though KVM Userfault today doesn't scale very
well).

Signed-off-by: James Houghton <jthoughton@...gle.com>
---
 arch/arm64/kvm/mmu.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index ac283e606516..c84633c9ab98 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -1488,6 +1488,15 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
 	mmu_seq = vcpu->kvm->mmu_invalidate_seq;
 	mmap_read_unlock(current->mm);
 
+	if (gfn_has_userfault(memslot->kvm, gfn)) {
+		kvm_prepare_memory_fault_exit(vcpu, gfn << PAGE_SHIFT,
+					      PAGE_SIZE, write_fault,
+					      /*exec=*/false,
+					      /*private=*/false,
+					      /*userfault=*/true);
+		return -EFAULT;
+	}
+
 	pfn = __gfn_to_pfn_memslot(memslot, gfn, false, false, NULL,
 				   write_fault, &writable, NULL);
 	if (pfn == KVM_PFN_ERR_HWPOISON) {
-- 
2.45.2.993.g49e7a77208-goog