linux-kernel - RE: [PATCH] PCI: hv: Fix multi-MSI to allow more than one MSI vector

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Date:   Tue, 12 Apr 2022 19:12:42 +0000
From:   Dexuan Cui <decui@...rosoft.com>
To:     Jeffrey Hugo <quic_jhugo@...cinc.com>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>,
        "lorenzo.pieralisi@....com" <lorenzo.pieralisi@....com>,
        "robh@...nel.org" <robh@...nel.org>, "kw@...ux.com" <kw@...ux.com>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        Jake Oshins <jakeo@...rosoft.com>
CC:     "bjorn.andersson@...aro.org" <bjorn.andersson@...aro.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] PCI: hv: Fix multi-MSI to allow more than one MSI vector

> From: Jeffrey Hugo <quic_jhugo@...cinc.com>
> Sent: Tuesday, April 12, 2022 7:17 AM
>  ...
> 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().
> 
> Fixes: 4daace0d8ce8 ("PCI: hv: Add paravirtual PCI front-end for Microsoft
> Hyper-V VMs")
> Signed-off-by: Jeffrey Hugo <quic_jhugo@...cinc.com>

Thanks for the fix! This looks good to me.

Reviewed-by: Dexuan Cui <decui@...rosoft.com>