linux-kernel - Re: [patch V2 30/46] x86/xen: Wrap XEN MSI management into irqdomain

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Date:   Sun, 15 Jan 2023 20:27:44 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        xen-devel <xen-devel@...ts.xen.org>
Cc:     Dimitri Sivanich <sivanich@....com>, linux-hyperv@...r.kernel.org,
        Steve Wahl <steve.wahl@....com>, linux-pci@...r.kernel.org,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Wei Liu <wei.liu@...nel.org>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Baolu Lu <baolu.lu@...el.com>, Marc Zyngier <maz@...nel.org>,
        x86@...nel.org, Jason Gunthorpe <jgg@...lanox.com>,
        Megha Dey <megha.dey@...el.com>,
        xen-devel@...ts.xenproject.org, Kevin Tian <kevin.tian@...el.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Jon Derrick <jonathan.derrick@...el.com>,
        Juergen Gross <jgross@...e.com>, Russ Anderson <rja@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        iommu@...ts.linux-foundation.org,
        Jacob Pan <jacob.jun.pan@...el.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>
Subject: Re: [patch V2 30/46] x86/xen: Wrap XEN MSI management into irqdomain

On Sun, 2023-01-15 at 14:12 +0000, David Woodhouse wrote:
> On Wed, 2020-08-26 at 13:16 +0200, Thomas Gleixner wrote:
> > From: Thomas Gleixner <tglx@...utronix.de>
> > 
> > To allow utilizing the irq domain pointer in struct device it is necessary
> > to make XEN/MSI irq domain compatible.
> > 
> > While the right solution would be to truly convert XEN to irq domains, this
> > is an exercise which is not possible for mere mortals with limited XENology.
> > 
> > Provide a plain irqdomain wrapper around XEN. While this is blatant
> > violation of the irqdomain design, it's the only solution for a XEN igorant
> > person to make progress on the issue which triggered this change.
> > 
> > Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> > Acked-by: Juergen Gross <jgross@...e.com>
> 
> I think it broke MSI-X support, because xen_pci_msi_domain_info is
> lacking a .flags = MSI_FLAGS_PCI_MSIX?

Hm, I think it only actually *broke* with commit 99f3d27976 ("PCI/MSI:
Reject MSI-X early") from November last year. So 6.1 was OK and we have
time to fix it in 6.2.

Confirmed on real Xen at least that a Fedora 37 install with a 6.0.7
kernel works fine, then on upgrading to Rawhide's 6.2-rc3 it dies with 

[   41.498694] ena 0000:00:03.0 (unnamed net_device) (uninitialized): Failed to enable MSI-X. irq_cnt -524
[   41.498705] ena 0000:00:03.0: Can not reserve msix vectors
[   41.498712] ena 0000:00:03.0: Failed to enable and set the admin interrupts

> > ---
> > Note: This is completely untested, but it compiles so it must be perfect.
> 
> 
> I'm working on making it simple for you to test that, by hosting Xen
> HVM guests natively in qemu (under KVM¹). 
> 
> But I'm absolutely not going to try hacking on both guest and host side
> at the same time when I'm trying to ensure compatibility — that way
> lies madness.
> 
> So for now I'm going to test qemu with older kernels, and maybe someone
> (Jürgen}? can test MSI-X to PIRQ support under real Xen?) FWIW if I add
> the missing MSI_FLAGS_PCI_MSIX flag then under my qemu I get:
> 
>  38:       3180          0  xen-pirq    -msi-x     ens4-rx-0
>  39:          0       3610  xen-pirq    -msi-x     ens4-tx-0
>  40:          1          0  xen-pirq    -msi-x     ens4
> 
> But without the flags I get:
> 
> [    8.464212] e1000e 0000:00:04.0 ens4: Failed to initialize MSI interrupts.  Falling back to legacy interrupts.
> 
> ¹ https://lore.kernel.org/qemu-devel/20230110122042.1562155-1-dwmw2@infradead.org/


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