linux-kernel - Re: [PATCH v2] PCI: hv: Fix multi-MSI to allow more than one MSI vector

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Date:   Mon, 25 Apr 2022 15:49:55 +0000
From:   Wei Liu <wei.liu@...nel.org>
To:     Jeffrey Hugo <quic_jhugo@...cinc.com>
Cc:     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, bjorn.andersson@...aro.org,
        linux-hyperv@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] PCI: hv: Fix multi-MSI to allow more than one MSI
 vector

On Mon, Apr 25, 2022 at 03:33:44PM +0000, Wei Liu wrote:
> On Wed, Apr 20, 2022 at 08:13:22AM -0600, Jeffrey Hugo wrote:
> > On 4/13/2022 7:36 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>
> > > Reviewed-by: Dexuan Cui <decui@...rosoft.com>
> > > ---
> > 
> > Ping?
> > 
> > I don't see this in -next, nor have I seen any replies.  It is possible I
> > have missed some kind of update, but currently I'm wondering if this change
> > is progressing or not.  If there is some kind of process used in this area,
> > I'm not familiar with it, so I would appreciate an introduction.
> 
> I expect the PCI maintainers to pick this up. If I don't see this picked
> up in this week I will apply it to hyperv-next.

Actually I will pick this up via hyperv-next, because there is another
series which will also touch this driver but at the some time depend on
vmbus changes. I can fix up any potential conflicts easily.

Thanks,
Wei.