linux-kernel - Re: [PATCH v3 3/3] PCI/MSI: Convert pci_msi_ignore

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Message-ID: <Z-PhgWQMHjxbac3b@macbook.local>
Date: Wed, 26 Mar 2025 12:14:09 +0100
From: Roger Pau Monné <roger.pau@...rix.com>
To: Borislav Petkov <bp@...en8.de>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org,
	linux-pci@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Juergen Gross <jgross@...e.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>, Ingo Molnar <mingo@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v3 3/3] PCI/MSI: Convert pci_msi_ignore_mask to per MSI
 domain flag

On Wed, Mar 26, 2025 at 12:04:55PM +0100, Borislav Petkov wrote:
> + Linus so that he can whack it before it spreads any further.
> 
> On Wed, Feb 19, 2025 at 10:20:57AM +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.
> > 
> > Albeit Devices behind a VMD bridge are not known to Xen, that doesn't mean
> > Linux cannot use them.  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.
> > 
> > Signed-off-by: Roger Pau Monné <roger.pau@...rix.com>
> > Reviewed-by: Thomas Gleixner <tglx@...utronix.de>
> > Acked-by: Juergen Gross <jgross@...e.com>
> 
> Did anyone actually test this on a normal KVM guest?

Sorry, not on KVM, I've tested on Xen and native.  It also seems to be
somewhat tied to the Kconfig, as I couldn't reproduce it with my
Kconfig, maybe didn't have the required VirtIO options enabled.

It's fixed by:

https://lore.kernel.org/xen-devel/87v7rxzct0.ffs@tglx/

Waiting for Thomas to formally sent that.

Thanks, Roger.