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Message-ID: <1421077333.6130.21.camel@redhat.com>
Date: Mon, 12 Jan 2015 08:42:13 -0700
From: Alex Williamson <alex.williamson@...hat.com>
To: Bjorn Helgaas <bhelgaas@...gle.com>
Cc: linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
x86@...nel.org, tglx@...utronix.de, mingo@...hat.com,
hpa@...or.com, joro@...tes.org, iommu@...ts.linux-foundation.org
Subject: Re: [PATCH 0/3] PCI/x86: Interface for testing multivector MSI
support
On Thu, 2015-01-08 at 09:15 -0700, Bjorn Helgaas wrote:
> On Fri, Nov 21, 2014 at 03:08:27PM -0700, Alex Williamson wrote:
> > I'd like to make vfio-pci capable of manipulating the device exposed
> > to the user such that if the host can only support a single MSI
> > vector then we hide the fact that the device itself may actually be
> > able to support more. When we virtualize PCI config space and
> > interrupt setup there's no PCI protocol for the device failing to
> > allocate the number of vectors that it said were available. If the
> > userspace driver is a guest operating system, it certainly doesn't
> > expect this to fail. I don't think we can ever guarantee that a
> > multi-vector request will succeed, but we can certainly guarantee
> > that it will fail if the platform doesn't support it.
> >
> > An example device is the Atheros AR93xxx running in a Windows 7 VM.
> > Both the device and the guest OS support multiple MSI vectors. With
> > interrupt remapping, such that the host supports multivector, the
> > device works well in the guest. With interrupt remapping disabled,
> > the device is far less reliable because of the mismatch in MSI
> > programming vs driver configuration and often fails. If vfio-pci
> > can test whether multiple vectors are supported, then we can make it
> > work reliably in both cases by adjusting the exposed MSI capability,
> > like in this patch that would follow this series:
> >
> > https://github.com/awilliam/linux-vfio/commit/9ace67515680
> >
> > With this series, only x86 w/ interrupt remapping will advertise
> > support for multiple MSI vectors. In surveying the code, I couldn't
> > find any other archs that allowed it, but I'll take corrections if
> > that's untrue. Thanks,
>
> Per Thomas' comments and your possible workaround if we don't have
> pci_msi_supported(), I'm going to ignore these for now. Let me know if
> you disagree.
Yep, that's fine. I'll either forget about this for a while or kludge
something in vfio to know that only x86 with interrupt remapping, which
I can test from the IOMMU API, has multivector MSI support. Thanks,
Alex
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