linux-kernel - Re: [PATCH 3/3] pci/msi: remove pci_msi_ignore

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Message-ID: <Z4TqNn_RSwGX1TQn@macbook.local>
Date: Mon, 13 Jan 2025 11:25:58 +0100
From: Roger Pau Monné <roger.pau@...rix.com>
To: Bjorn Helgaas <helgaas@...nel.org>
Cc: linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org,
	linux-pci@...r.kernel.org, Juergen Gross <jgross@...e.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 3/3] pci/msi: remove pci_msi_ignore_mask

On Fri, Jan 10, 2025 at 04:30:57PM -0600, Bjorn Helgaas wrote:
> Match subject line style again.
> 
> On Fri, Jan 10, 2025 at 03:01:50PM +0100, Roger Pau Monne wrote:
> > Setting pci_msi_ignore_mask inhibits the toggling of the mask bit for both MSI
> > and MSI-X entries globally, regardless of the IRQ chip they are using.  Only
> > Xen sets the pci_msi_ignore_mask when routing physical interrupts over event
> > channels, to prevent PCI code from attempting to toggle the maskbit, as it's
> > Xen that controls the bit.
> > 
> > However, the pci_msi_ignore_mask being global will affect devices that use MSI
> > interrupts but are not routing those interrupts over event channels (not using
> > the Xen pIRQ chip).  One example is devices behind a VMD PCI bridge.  In that
> > scenario the VMD bridge configures MSI(-X) using the normal IRQ chip (the pIRQ
> > one in the Xen case), and devices behind the bridge configure the MSI entries
> > using indexes into the VMD bridge MSI table.  The VMD bridge then demultiplexes
> > such interrupts and delivers to the destination device(s).  Having
> > pci_msi_ignore_mask set in that scenario prevents (un)masking of MSI entries
> > for devices behind the VMD bridge.
> > 
> > Move the signaling of no entry masking into the MSI domain flags, as that
> > allows setting it on a per-domain basis.  Set it for the Xen MSI domain that
> > uses the pIRQ chip, while leaving it unset for the rest of the cases.
> > 
> > Remove pci_msi_ignore_mask at once, since it was only used by Xen code, and
> > with Xen dropping usage the variable is unneeded.
> > 
> > This fixes using devices behind a VMD bridge on Xen PV hardware domains.
> 
> Wrap to fit in 75 columns.
> 
> The first two patches talk about devices behind VMD not being usable
> for Xen, but this one says they now work.

Sorry, let me try to clarify:

Devices behind a VMD bridge are not known to Xen, however that doesn't
mean Linux cannot use them.  That's what this series achieves.  By
inhibiting the usage of VMD_FEAT_CAN_BYPASS_MSI_REMAP and the removal
of the pci_msi_ignore_mask bodge devices behind a VMD bridge do work
fine when use from a Linux Xen hardware domain.  That's the whole
point of the series.

> But this doesn't undo the
> code changes or comments added by the first two, so the result is a
> bit confusing (probably because I know nothing about Xen).

All patches are needed.  There's probably some confusion about devices
behind a VMD bridge not being known by Xen vs not being usable by
Linux running under Xen as a hardware domain.

With all three patches applied devices behind a VMD bridge work under
Linux with Xen.

Thanks, Roger.