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: <20250729142917.GF26511@ziepe.ca>
Date: Tue, 29 Jul 2025 11:29:17 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>
Cc: 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>,
	Xu Yilun <yilun.xu@...ux.intel.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 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,
if it is global then vfio can pick it up during the early phases of
opening the device.

> IIUC, we intend to request the resource in exclusive mode for secure
> guests—regardless of whether the platform is Intel or ARM. Could you
> help clarify the MCE issue observed on Intel platforms in this context?

As I understand it Intel MCEs if the non-secure side ever reads from
secure'd address space. So there is alot of emphasis there to ensure
there are no CPU mappings.

Jason