linux-kernel - [PATCH v2 1/2] PCI: hv: Reuse existing IRTE allocation in compose_msi

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Date:   Wed, 11 May 2022 09:23:02 -0600
From:   Jeffrey Hugo <quic_jhugo@...cinc.com>
To:     <kys@...rosoft.com>, <haiyangz@...rosoft.com>,
        <sthemmin@...rosoft.com>, <wei.liu@...nel.org>,
        <decui@...rosoft.com>, <lorenzo.pieralisi@....com>,
        <robh@...nel.org>, <kw@...ux.com>, <bhelgaas@...gle.com>
CC:     <jakeo@...rosoft.com>, <dazhan@...rosoft.com>,
        <linux-hyperv@...r.kernel.org>, <linux-pci@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        Jeffrey Hugo <quic_jhugo@...cinc.com>
Subject: [PATCH v2 1/2] PCI: hv: Reuse existing IRTE allocation in compose_msi_msg()

Currently if compose_msi_msg() is called multiple times, it will free any
previous IRTE allocation, and generate a new allocation.  While nothing
prevents this from occurring, it is extraneous when Linux could just reuse
the existing allocation and avoid a bunch of overhead.

However, when future IRTE allocations operate on blocks of MSIs instead of
a single line, freeing the allocation will impact all of the lines.  This
could cause an issue where an allocation of N MSIs occurs, then some of
the lines are retargeted, and finally the allocation is freed/reallocated.
The freeing of the allocation removes all of the configuration for the
entire block, which requires all the lines to be retargeted, which might
not happen since some lines might already be unmasked/active.

Signed-off-by: Jeffrey Hugo <quic_jhugo@...cinc.com>
Reviewed-by: Dexuan Cui <decui@...rosoft.com>
Tested-by: Dexuan Cui <decui@...rosoft.com>
Tested-by: Michael Kelley <mikelley@...rosoft.com>
---
 drivers/pci/controller/pci-hyperv.c | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/drivers/pci/controller/pci-hyperv.c b/drivers/pci/controller/pci-hyperv.c
index cf2fe575..5e2e637 100644
--- a/drivers/pci/controller/pci-hyperv.c
+++ b/drivers/pci/controller/pci-hyperv.c
@@ -1707,6 +1707,15 @@ static void hv_compose_msi_msg(struct irq_data *data, struct msi_msg *msg)
 	u32 size;
 	int ret;
 
+	/* Reuse the previous allocation */
+	if (data->chip_data) {
+		int_desc = data->chip_data;
+		msg->address_hi = int_desc->address >> 32;
+		msg->address_lo = int_desc->address & 0xffffffff;
+		msg->data = int_desc->data;
+		return;
+	}
+
 	pdev = msi_desc_to_pci_dev(irq_data_get_msi_desc(data));
 	dest = irq_data_get_effective_affinity_mask(data);
 	pbus = pdev->bus;
@@ -1716,13 +1725,6 @@ 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) {
-		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;
-- 
2.7.4