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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org, stable@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jeffrey Hugo <quic_jhugo@quicinc.com>,
        Dexuan Cui <decui@microsoft.com>, Wei Liu <wei.liu@kernel.org>,
        Carl Vanderlip <quic_carlv@quicinc.com>
Subject: [PATCH 5.10 017/105] PCI: hv: Fix multi-MSI to allow more than one MSI vector
Date:   Wed, 27 Jul 2022 18:10:03 +0200
Message-Id: <20220727161012.776500218@linuxfoundation.org>
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From: Jeffrey Hugo <quic_jhugo@quicinc.com>

commit 08e61e861a0e47e5e1a3fb78406afd6b0cea6b6d upstream.

If the allocation of multiple MSI vectors for multi-MSI fails in the core
PCI framework, the framework will retry the allocation as a single MSI
vector, assuming that meets the min_vecs specified by the requesting
driver.

Hyper-V advertises that multi-MSI is supported, but reuses the VECTOR
domain to implement that for x86.  The VECTOR domain does not support
multi-MSI, so the alloc will always fail and fallback to a single MSI
allocation.

In short, Hyper-V advertises a capability it does not implement.

Hyper-V can support multi-MSI because it coordinates with the hypervisor
to map the MSIs in the IOMMU's interrupt remapper, which is something the
VECTOR domain does not have.  Therefore the fix is simple - copy what the
x86 IOMMU drivers (AMD/Intel-IR) do by removing
X86_IRQ_ALLOC_CONTIGUOUS_VECTORS after calling the VECTOR domain's
pci_msi_prepare().

5.10 backport - adds the hv_msi_prepare wrapper function

Fixes: 4daace0d8ce8 ("PCI: hv: Add paravirtual PCI front-end for Microsoft Hyper-V VMs")
Signed-off-by: Jeffrey Hugo <quic_jhugo@quicinc.com>
Reviewed-by: Dexuan Cui <decui@microsoft.com>
Link: https://lore.kernel.org/r/1649856981-14649-1-git-send-email-quic_jhugo@quicinc.com
Signed-off-by: Wei Liu <wei.liu@kernel.org>
Signed-off-by: Carl Vanderlip <quic_carlv@quicinc.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/pci/controller/pci-hyperv.c |   17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

--- a/drivers/pci/controller/pci-hyperv.c
+++ b/drivers/pci/controller/pci-hyperv.c
@@ -1180,6 +1180,21 @@ static void hv_irq_mask(struct irq_data
 	pci_msi_mask_irq(data);
 }
 
+static int hv_msi_prepare(struct irq_domain *domain, struct device *dev,
+			  int nvec, msi_alloc_info_t *info)
+{
+	int ret = pci_msi_prepare(domain, dev, nvec, info);
+
+	/*
+	 * By using the interrupt remapper in the hypervisor IOMMU, contiguous
+	 * CPU vectors is not needed for multi-MSI
+	 */
+	if (info->type == X86_IRQ_ALLOC_TYPE_PCI_MSI)
+		info->flags &= ~X86_IRQ_ALLOC_CONTIGUOUS_VECTORS;
+
+	return ret;
+}
+
 /**
  * hv_irq_unmask() - "Unmask" the IRQ by setting its current
  * affinity.
@@ -1545,7 +1560,7 @@ static struct irq_chip hv_msi_irq_chip =
 };
 
 static struct msi_domain_ops hv_msi_ops = {
-	.msi_prepare	= pci_msi_prepare,
+	.msi_prepare	= hv_msi_prepare,
 	.msi_free	= hv_msi_free,
 };