linux-kernel - Re: [RFC PATCH v1 06/38] iommufd: Add and option to request for bar mapping with IORESOURCE

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Message-ID: <20250731122217.GR26511@ziepe.ca>
Date: Thu, 31 Jul 2025 09:22:17 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Xu Yilun <yilun.xu@...ux.intel.com>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>,
	linux-coco@...ts.linux.dev, kvmarm@...ts.linux.dev,
	linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
	aik@....com, lukas@...ner.de, Samuel Ortiz <sameo@...osinc.com>,
	Suzuki K Poulose <Suzuki.Poulose@....com>,
	Steven Price <steven.price@....com>,
	Catalin Marinas <catalin.marinas@....com>,
	Marc Zyngier <maz@...nel.org>, Will Deacon <will@...nel.org>,
	Oliver Upton <oliver.upton@...ux.dev>
Subject: Re: [RFC PATCH v1 06/38] iommufd: Add and option to request for bar
 mapping with IORESOURCE_EXCLUSIVE

On Wed, Jul 30, 2025 at 02:55:02PM +0800, Xu Yilun wrote:
> On Tue, Jul 29, 2025 at 11:29:17AM -0300, Jason Gunthorpe wrote:
> > On Tue, Jul 29, 2025 at 01:58:54PM +0530, Aneesh Kumar K.V wrote:
> > > Jason Gunthorpe <jgg@...pe.ca> writes:
> > > 
> > > > On Mon, Jul 28, 2025 at 07:21:43PM +0530, Aneesh Kumar K.V (Arm) wrote:
> > > >> Signed-off-by: Aneesh Kumar K.V (Arm) <aneesh.kumar@...nel.org>
> > > >
> > > > Why would we need this?
> > > >
> > > > I can sort of understand why Intel would need it due to their issues
> > > > with MCE, but ARM shouldn't care either way, should it?
> > > >
> > > > But also why is it an iommufd option? That doesn't seem right..
> > > >
> > > > Jason
> > > 
> > > This is based on our previous discussion https://lore.kernel.org/all/20250606120919.GH19710@nvidia.com
> > 
> > I suggested a global option, this is a per-device option, and that
> > especially seems wrong for iommufd. If it is per-device that is vfio,
> 
> I think this should be per-device.

IMHO there is no use case for that, it should arguably be global to
the whole kernel.

> The original purpose of this pci_region_request_*() is to prevent
> further mmap/read/write against a vfio_cdev FD which would be used

No way, the VFIO internal mmap should be controled by VFIO not by
request region. If you want to block that it should be blocked by
iommufd telling VFIO that the device is bound which revokes the
mmaps/dmabufs/etc and prevents opening new ones.

The only thing request region should do is prevent /sys/../resource,
/dev/mem users and so on, which is why it can and should be
global. Arguably VFIO should always block those things but
historically hasn't..

There should only be one request region call in VFIO, it should
ideally happen when the VFIO driver probes the device.

Jason