linux-kernel - Re: [PATCH v5 0/2] vfio/pci: add msi interrupt affinity support

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Date: Wed, 26 Jun 2024 13:49:38 +0200
From: Michal Koutný <mkoutny@...e.com>
To: Frederic Griffoul <griffoul@...il.com>
Cc: Fred Griffoul <fgriffo@...zon.co.uk>, 
	Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>, 
	Alex Williamson <alex.williamson@...hat.com>, Waiman Long <longman@...hat.com>, 
	Zefan Li <lizefan.x@...edance.com>, Tejun Heo <tj@...nel.org>, Johannes Weiner <hannes@...xchg.org>, 
	Mark Rutland <mark.rutland@....com>, Marc Zyngier <maz@...nel.org>, 
	Oliver Upton <oliver.upton@...ux.dev>, Mark Brown <broonie@...nel.org>, 
	Ard Biesheuvel <ardb@...nel.org>, Joey Gouly <joey.gouly@....com>, 
	Ryan Roberts <ryan.roberts@....com>, Jeremy Linton <jeremy.linton@....com>, 
	Jason Gunthorpe <jgg@...pe.ca>, Yi Liu <yi.l.liu@...el.com>, Kevin Tian <kevin.tian@...el.com>, 
	Eric Auger <eric.auger@...hat.com>, Stefan Hajnoczi <stefanha@...hat.com>, 
	Christian Brauner <brauner@...nel.org>, Ankit Agrawal <ankita@...dia.com>, 
	Reinette Chatre <reinette.chatre@...el.com>, Ye Bin <yebin10@...wei.com>, 
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org, 
	cgroups@...r.kernel.org
Subject: Re: [PATCH v5 0/2] vfio/pci: add msi interrupt affinity support

On Wed, Jun 12, 2024 at 03:45:48PM GMT, Frederic Griffoul <griffoul@...il.com> wrote:
> To be honest my initial idea was to store an affinity mask per vfio group, which
> can be done in the privileged process setting the vfio group/device owner, and
> later apply the mask to each interrupt of each device in the group.
> 
> It would still require to fix the affinity of all the interrupts if
> the vfio group affinity is
> changed (or deliberately ignore this case). And it did not match
> exactly my use case
> where I need the process handling the interrupts to sometimes be able
> to change them
> but always within the cpuset. So I would still need the current patch,
> in addition to
> a new ioctl() to set the affinity mask of a vfio group.

It's not clear to me what is the relation between the process A calling
that ioctl() (and whose cpuset is used to check affinity for)
and a process B that ends up handling the IRQ.
At which place would process B be scheduled on IRQ's affinity CPUs?
(I'm not familiar with VFIO.)

Thanks,
Michal

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