PCI: hv: Only reuse existing IRTE allocation for Multi-MSI

Jeffrey added Multi-MSI support to the pci-hyperv driver by the 4 patches:
08e61e861a0e ("PCI: hv: Fix multi-MSI to allow more than one MSI vector") 455880dfe292 ("PCI: hv: Fix hv_arch_irq_unmask() for multi-MSI") b4b77778ecc5 ("PCI: hv: Reuse existing IRTE allocation in compose_msi_msg()") a2bad844a67b ("PCI: hv: Fix interrupt mapping for multi-MSI")

It turns out that the third patch (b4b77778ecc5) causes a performance regression because all the interrupts now happen on 1 physical CPU (or two pCPUs, if one pCPU doesn't have enough vectors). When a guest has many PCI devices, it may suffer from soft lockups if the workload is heavy, e.g., see https://lwn.net/ml/linux-kernel/[email protected]/

Commit b4b77778ecc5 itself is good. The real issue is that the hypercall in hv_irq_unmask() -> hv_arch_irq_unmask() -> hv_do_hypercall(HVCALL_RETARGET_INTERRUPT...) only changes the target virtual CPU rather than physical CPU; with b4b77778ecc5, the pCPU is determined only once in hv_compose_msi_msg() where only vCPU0 is specified; consequently the hypervisor only uses 1 target pCPU for all the interrupts.

Note: before b4b77778ecc5, the pCPU is determined twice, and when the pCPU is determinted the second time, the vCPU in the effective affinity mask is used (i.e., it isn't always vCPU0), so the hypervisor chooses a different pCPU for each interrupt.

The hypercall will be fixed in future to update the pCPU as well, but that will take quite a while, so let's restore the old behavior in hv_compose_msi_msg(), i.e., don't reuse the existing IRTE allocation for single-MSI and MSI-X; for multi-MSI, we choose the vCPU in a round-robin manner for each PCI device, so the interrupts of different devices can happen on different pCPUs, though the interrupts of each device happen on some single pCPU.

The hypercall fix may not be backported to all old versions of Hyper-V, so we want to have this guest side change for ever (or at least till we're sure the old affected versions of Hyper-V are no longer supported).

Fixes b4b77778ecc5 ("PCI: hv: Reuse existing IRTE allocation in compose_msi_msg()")

Co-developed-by Jeffrey Hugo <[email protected]>

Signed-off-by Jeffrey Hugo <[email protected]>

Co-developed-by Carl Vanderlip <[email protected]>

Signed-off-by Carl Vanderlip <[email protected]>

Signed-off-by Dexuan Cui <[email protected]>

---
v1 is here:

https://lwn.net/ml/linux-kernel/[email protected]/

Changes in v2:
round-robin the vCPU for multi-MSI.
The commit message is re-worked.
Added Jeff and Carl's Co-developed-by and Signed-off-by.

 drivers/pci/controller/pci-hyperv.c | 61 ++++++++++++++++++++++-------
 1 file changed, 46 insertions(+), 15 deletions(-)

drivers/pci/controller/pci-hyperv.c +1 -1

index e7c6f6629e7c..7ac080f95259 100644
@@ -1613,7 +1613,7 @@ static void hv_pci_compose_compl(void *context, struct
 pci_response *resp,
 }
 
 static u32 hv_compose_msi_req_v1(
-	struct pci_create_interrupt *int_pkt, const struct cpumask *affinity,
+	struct pci_create_interrupt *int_pkt,
 	u32 slot, u8 vector, u8 vector_count)
 {

drivers/pci/controller/pci-hyperv.c +6 -3

 	int_pkt->message_type.type = PCI_CREATE_INTERRUPT_MESSAGE;

@@ -1640,18 +1640,39 @@ static int hv_compose_msi_req_get_cpu(const struct cpumask *affinity)
 	return cpumask_first_and(affinity, cpu_online_mask);
 }
 
-static u32 hv_compose_msi_req_v2(
-	struct pci_create_interrupt2 *int_pkt, const struct cpumask *affinity,
-	u32 slot, u8 vector, u8 vector_count)
+/*
+ * Make sure the dummy vCPU values for multi-MSI don't all point to vCPU0.
+ */
+static int hv_compose_multi_msi_req_get_cpu(void)
 {
+	static DEFINE_SPINLOCK(multi_msi_cpu_lock);
+

drivers/pci/controller/pci-hyperv.c +31 -12

+	cpu = cpu_next;
+
+	spin_unlock_irqrestore(&multi_msi_cpu_lock, flags);
+
+	return cpu;
+}
+
+static u32 hv_compose_msi_req_v2(
+	struct pci_create_interrupt2 *int_pkt, int cpu,
+	u32 slot, u8 vector, u8 vector_count)
+{
 	int_pkt->message_type.type = PCI_CREATE_INTERRUPT_MESSAGE2;
 	int_pkt->wslot.slot = slot;
 	int_pkt->int_desc.vector = vector;
 	int_pkt->int_desc.vector_count = vector_count;
 	int_pkt->int_desc.delivery_mode = DELIVERY_MODE;
-	cpu = hv_compose_msi_req_get_cpu(affinity);
 	int_pkt->int_desc.processor_array[0] =
 		hv_cpu_number_to_vp_number(cpu);
 	int_pkt->int_desc.processor_count = 1;

@@ -1660,18 +1681,15 @@ static u32 hv_compose_msi_req_v2(
 }
 
 static u32 hv_compose_msi_req_v3(
-	struct pci_create_interrupt3 *int_pkt, const struct cpumask *affinity,
+	struct pci_create_interrupt3 *int_pkt, int cpu,
 	u32 slot, u32 vector, u8 vector_count)
 {
-	int cpu;
-
 	int_pkt->message_type.type = PCI_CREATE_INTERRUPT_MESSAGE3;
 	int_pkt->wslot.slot = slot;
 	int_pkt->int_desc.vector = vector;
 	int_pkt->int_desc.reserved = 0;
 	int_pkt->int_desc.vector_count = vector_count;
 	int_pkt->int_desc.delivery_mode = DELIVERY_MODE;
-	cpu = hv_compose_msi_req_get_cpu(affinity);
 	int_pkt->int_desc.processor_array[0] =
 		hv_cpu_number_to_vp_number(cpu);
 	int_pkt->int_desc.processor_count = 1;

@@ -1710,12 +1728,18 @@ static void hv_compose_msi_msg(struct irq_data *data, struct msi_msg *msg)
 			struct pci_create_interrupt3 v3;
 		} int_pkts;
 	} __packed ctxt;
+	bool multi_msi;
 	u64 trans_id;
 	u32 size;
 	int ret;
+	int cpu;
+
+	msi_desc  = irq_data_get_msi_desc(data);
+	multi_msi = !msi_desc->pci.msi_attrib.is_msix &&
+		    msi_desc->nvec_used > 1;
 
 	/* Reuse the previous allocation */
-	if (data->chip_data) {
+	if (data->chip_data && multi_msi) {
 		int_desc = data->chip_data;
 		msg->address_hi = int_desc->address >> 32;
 		msg->address_lo = int_desc->address & 0xffffffff;

@@ -1723,7 +1747,6 @@ static void hv_compose_msi_msg(struct irq_data *data, struct msi_msg *msg)
 		return;
 	}
 
-	msi_desc  = irq_data_get_msi_desc(data);
 	pdev = msi_desc_to_pci_dev(msi_desc);
 	dest = irq_data_get_effective_affinity_mask(data);
 	pbus = pdev->bus;

@@ -1733,11 +1756,18 @@ static void hv_compose_msi_msg(struct irq_data *data, struct msi_msg *msg)
 	if (!hpdev)
 		goto return_null_message;
 
+	/* Free any previous message that might have already been composed. */
+	if (data->chip_data && !multi_msi) {
+		int_desc = data->chip_data;
+		data->chip_data = NULL;
+		hv_int_desc_free(hpdev, int_desc);
+	}
+
 	int_desc = kzalloc(sizeof(*int_desc), GFP_ATOMIC);
 	if (!int_desc)
 		goto drop_reference;
 
-	if (!msi_desc->pci.msi_attrib.is_msix && msi_desc->nvec_used > 1) {
+	if (multi_msi) {
 		/*
 		 * If this is not the first MSI of Multi MSI, we already have
 		 * a mapping.  Can exit early.

@@ -1762,9 +1792,11 @@ static void hv_compose_msi_msg(struct irq_data *data, struct msi_msg *msg)
 		 */
 		vector = 32;
 		vector_count = msi_desc->nvec_used;
+		cpu = hv_compose_multi_msi_req_get_cpu();
 	} else {
 		vector = hv_msi_get_int_vector(data);
 		vector_count = 1;
+		cpu = hv_compose_msi_req_get_cpu(dest);
 	}
 
 	memset(&ctxt, 0, sizeof(ctxt));

@@ -1775,7 +1807,6 @@ static void hv_compose_msi_msg(struct irq_data *data, struct msi_msg *msg)
 	switch (hbus->protocol_version) {
 	case PCI_PROTOCOL_VERSION_1_1:
 		size = hv_compose_msi_req_v1(&ctxt.int_pkts.v1,
-					dest,
 					hpdev->desc.win_slot.slot,
 					vector,
 					vector_count);

@@ -1784,7 +1815,7 @@ static void hv_compose_msi_msg(struct irq_data *data, struct msi_msg *msg)
 	case PCI_PROTOCOL_VERSION_1_2:
 	case PCI_PROTOCOL_VERSION_1_3:
 		size = hv_compose_msi_req_v2(&ctxt.int_pkts.v2,
-					dest,
+					cpu,
 					hpdev->desc.win_slot.slot,
 					vector,
 					vector_count);

@@ -1792,7 +1823,7 @@ static void hv_compose_msi_msg(struct irq_data *data, struct msi_msg *msg)
 
 	case PCI_PROTOCOL_VERSION_1_4:
 		size = hv_compose_msi_req_v3(&ctxt.int_pkts.v3,
-					dest,
+					cpu,
 					hpdev->desc.win_slot.slot,
 					vector,
 					vector_count);
--

2.25.1

From: Michael Kelley (LINUX) <[email protected]>
Sent: Tuesday, October 25, 2022 11:18 AM

Quoted 10 lines

...

drivers/pci/controller/pci-hyperv.c +5 -3

-static u32 hv_compose_msi_req_v2(
-	struct pci_create_interrupt2 *int_pkt, const struct cpumask *affinity,
-	u32 slot, u8 vector, u8 vector_count)
+/*
+ * Make sure the dummy vCPU values for multi-MSI don't all point to vCPU0.
+ */
+static int hv_compose_multi_msi_req_get_cpu(void)
 {
+	static DEFINE_SPINLOCK(multi_msi_cpu_lock);

drivers/pci/controller/pci-hyperv.c static int hv_compose_msi_req_get_cpu(const struct cpumask *affinity)

Quoted 10 lines

drivers/pci/controller/pci-hyperv.c +8 -0

+
+	/* -1 means starting with CPU 0 */
+	static int cpu_next = -1;
+
+	unsigned long flags;
 	int cpu;
 
+	spin_lock_irqsave(&multi_msi_cpu_lock, flags);
+
+	cpu_next = cpumask_next_wrap(cpu_next, cpu_online_mask,

nr_cpu_ids,

drivers/pci/controller/pci-hyperv.c static int hv_compose_msi_req_get_cpu(const struct cpumask *affinity) · line 1659

Quoted 1 line

drivers/pci/controller/pci-hyperv.c +1 -0

+				     false);

I have a modest concern about using cpu_online_mask. The CPUs in this mask
can change if a CPU is taken online or offline in Linux. I don't think there's a requirement to pick on an online CPU, especially since if we pick a CPU that's online now, it might not be online later. Using cpu_present_mask would be more correct. That's the CPUs that are present in the VM, which won't change
over time since Hyper-V doesn't hot-add or hot-remove CPUs in a VM.

A similar concern applies to hv_compose_msi_req_get_cpu().

Michael