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 13:32:29 -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>,
        <jakeo@...rosoft.com>
CC:     <bjorn.andersson@...aro.org>, <linux-hyperv@...r.kernel.org>,
        <linux-pci@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] PCI: hv: Fix multi-MSI to allow more than one MSI vector

On 4/12/2022 8:16 AM, Jeffrey Hugo wrote:
> 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>
> ---
>   drivers/pci/controller/pci-hyperv.c | 11 ++++++++++-
>   1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/controller/pci-hyperv.c b/drivers/pci/controller/pci-hyperv.c
> index d270a204..41be63e 100644
> --- a/drivers/pci/controller/pci-hyperv.c
> +++ b/drivers/pci/controller/pci-hyperv.c
> @@ -614,7 +614,16 @@ static void hv_set_msi_entry_from_desc(union hv_msi_entry *msi_entry,
>   static int hv_msi_prepare(struct irq_domain *domain, struct device *dev,
>   			  int nvec, msi_alloc_info_t *info)
>   {
> -	return pci_msi_prepare(domain, dev, nvec, info);
> +	int ret = pci_msi_prepare(domain, dev, nvec, info);
> +
> +	/*
> +	 * By using the interrupt remapper in the hypervisor IOMMU, contiguous
> +	 * CPU vectors in not needed for multi-MSI

I just noticed that "in" should be "is".

> +	 */
> +	if (info->type == X86_IRQ_ALLOC_TYPE_PCI_MSI)
> +		info->flags &= ~X86_IRQ_ALLOC_CONTIGUOUS_VECTORS;
> +
> +	return ret;
>   }
>   
>   /**