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From: Suraj Jitindar Singh <sjitindarsingh@gmail.com>
To: kvm-ppc@vger.kernel.org
Cc: linuxppc-dev@lists.ozlabs.org,
	Suraj Jitindar Singh <sjitindarsingh@gmail.com>,
	kvm@vger.kernel.org
Subject: [PATCH] KVM: PPC: Book3S HV: Optimise mmio emulation for devices on FAST_MMIO_BUS
Date: Thu,  7 Feb 2019 15:56:50 +1100	[thread overview]
Message-ID: <20190207045650.18322-1-sjitindarsingh@gmail.com> (raw)

Devices on the KVM_FAST_MMIO_BUS by definition have length zero and are
thus used for notification purposes rather than data transfer. For
example eventfd for virtio devices.

This means that when emulating mmio instructions which target devices on
this bus we can immediately handle them and return without needing to load
the instruction from guest memory.

For now we restrict this to stores as this is the only use case at
present.

For a normal guest the effect is negligible, however for a nested guest
we save on the order of 5us per access.

Signed-off-by: Suraj Jitindar Singh <sjitindarsingh@gmail.com>
---
 arch/powerpc/kvm/book3s_64_mmu_hv.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/arch/powerpc/kvm/book3s_64_mmu_hv.c b/arch/powerpc/kvm/book3s_64_mmu_hv.c
index bd2dcfbf00cd..be7bc070eae5 100644
--- a/arch/powerpc/kvm/book3s_64_mmu_hv.c
+++ b/arch/powerpc/kvm/book3s_64_mmu_hv.c
@@ -442,6 +442,24 @@ int kvmppc_hv_emulate_mmio(struct kvm_run *run, struct kvm_vcpu *vcpu,
 	u32 last_inst;
 
 	/*
+	 * Fast path - check if the guest physical address corresponds to a
+	 * device on the FAST_MMIO_BUS, if so we can avoid loading the
+	 * instruction all together, then we can just handle it and return.
+	 */
+	if (is_store) {
+		int idx, ret;
+
+		idx = srcu_read_lock(&vcpu->kvm->srcu);
+		ret = kvm_io_bus_write(vcpu, KVM_FAST_MMIO_BUS, (gpa_t) gpa, 0,
+				       NULL);
+		srcu_read_unlock(&vcpu->kvm->srcu, idx);
+		if (!ret) {
+			kvmppc_set_pc(vcpu, kvmppc_get_pc(vcpu) + 4);
+			return RESUME_GUEST;
+		}
+	}
+
+	/*
 	 * If we fail, we just return to the guest and try executing it again.
 	 */
 	if (kvmppc_get_last_inst(vcpu, INST_GENERIC, &last_inst) !=
-- 
2.13.6

WARNING: multiple messages have this Message-ID (diff)
From: Suraj Jitindar Singh <sjitindarsingh@gmail.com>
To: kvm-ppc@vger.kernel.org
Cc: linuxppc-dev@lists.ozlabs.org,
	Suraj Jitindar Singh <sjitindarsingh@gmail.com>,
	kvm@vger.kernel.org
Subject: [PATCH] KVM: PPC: Book3S HV: Optimise mmio emulation for devices on FAST_MMIO_BUS
Date: Thu, 07 Feb 2019 04:56:50 +0000	[thread overview]
Message-ID: <20190207045650.18322-1-sjitindarsingh@gmail.com> (raw)

Devices on the KVM_FAST_MMIO_BUS by definition have length zero and are
thus used for notification purposes rather than data transfer. For
example eventfd for virtio devices.

This means that when emulating mmio instructions which target devices on
this bus we can immediately handle them and return without needing to load
the instruction from guest memory.

For now we restrict this to stores as this is the only use case at
present.

For a normal guest the effect is negligible, however for a nested guest
we save on the order of 5us per access.

Signed-off-by: Suraj Jitindar Singh <sjitindarsingh@gmail.com>
---
 arch/powerpc/kvm/book3s_64_mmu_hv.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/arch/powerpc/kvm/book3s_64_mmu_hv.c b/arch/powerpc/kvm/book3s_64_mmu_hv.c
index bd2dcfbf00cd..be7bc070eae5 100644
--- a/arch/powerpc/kvm/book3s_64_mmu_hv.c
+++ b/arch/powerpc/kvm/book3s_64_mmu_hv.c
@@ -442,6 +442,24 @@ int kvmppc_hv_emulate_mmio(struct kvm_run *run, struct kvm_vcpu *vcpu,
 	u32 last_inst;
 
 	/*
+	 * Fast path - check if the guest physical address corresponds to a
+	 * device on the FAST_MMIO_BUS, if so we can avoid loading the
+	 * instruction all together, then we can just handle it and return.
+	 */
+	if (is_store) {
+		int idx, ret;
+
+		idx = srcu_read_lock(&vcpu->kvm->srcu);
+		ret = kvm_io_bus_write(vcpu, KVM_FAST_MMIO_BUS, (gpa_t) gpa, 0,
+				       NULL);
+		srcu_read_unlock(&vcpu->kvm->srcu, idx);
+		if (!ret) {
+			kvmppc_set_pc(vcpu, kvmppc_get_pc(vcpu) + 4);
+			return RESUME_GUEST;
+		}
+	}
+
+	/*
 	 * If we fail, we just return to the guest and try executing it again.
 	 */
 	if (kvmppc_get_last_inst(vcpu, INST_GENERIC, &last_inst) !-- 
2.13.6

             reply	other threads:[~2019-02-07  4:56 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-02-07  4:56 Suraj Jitindar Singh [this message]
2019-02-07  4:56 ` [PATCH] KVM: PPC: Book3S HV: Optimise mmio emulation for devices on FAST_MMIO_BUS Suraj Jitindar Singh
2019-02-22  9:39 ` Paul Mackerras
2019-02-22  9:39   ` Paul Mackerras

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