linux-kernel - RE: [PATCH v8 09/11] vfio/pci: Enable peer-to-peer DMA transactions by default

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Message-ID: <BN9PR11MB5276E1C21E6F3CAA5E5C85068CD7A@BN9PR11MB5276.namprd11.prod.outlook.com>
Date: Wed, 19 Nov 2025 00:01:35 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Alex Williamson <alex@...zbot.org>
CC: Leon Romanovsky <leon@...nel.org>, Bjorn Helgaas <bhelgaas@...gle.com>,
	Logan Gunthorpe <logang@...tatee.com>, Jens Axboe <axboe@...nel.dk>, "Robin
 Murphy" <robin.murphy@....com>, Joerg Roedel <joro@...tes.org>, Will Deacon
	<will@...nel.org>, Marek Szyprowski <m.szyprowski@...sung.com>, "Jason
 Gunthorpe" <jgg@...pe.ca>, Andrew Morton <akpm@...ux-foundation.org>,
	Jonathan Corbet <corbet@....net>, Sumit Semwal <sumit.semwal@...aro.org>,
	Christian König <christian.koenig@....com>, Kees Cook
	<kees@...nel.org>, "Gustavo A. R. Silva" <gustavoars@...nel.org>, "Ankit
 Agrawal" <ankita@...dia.com>, Yishai Hadas <yishaih@...dia.com>, "Shameer
 Kolothum" <skolothumtho@...dia.com>, Krishnakant Jaju <kjaju@...dia.com>,
	Matt Ochs <mochs@...dia.com>, "linux-pci@...r.kernel.org"
	<linux-pci@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "linux-block@...r.kernel.org"
	<linux-block@...r.kernel.org>, "iommu@...ts.linux.dev"
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	"linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>, "Alex
 Mastro" <amastro@...com>, Nicolin Chen <nicolinc@...dia.com>
Subject: RE: [PATCH v8 09/11] vfio/pci: Enable peer-to-peer DMA transactions
 by default

> From: Alex Williamson <alex@...zbot.org>
> Sent: Wednesday, November 19, 2025 4:11 AM
> 
> On Tue, 18 Nov 2025 07:18:36 +0000
> "Tian, Kevin" <kevin.tian@...el.com> wrote:
> 
> > > From: Leon Romanovsky <leon@...nel.org>
> > > Sent: Tuesday, November 11, 2025 5:58 PM
> > >
> > > From: Leon Romanovsky <leonro@...dia.com>
> >
> > not required with only your own s-o-b
> >
> > > @@ -2090,6 +2092,9 @@ int vfio_pci_core_init_dev(struct vfio_device
> > > *core_vdev)
> > >  	INIT_LIST_HEAD(&vdev->dummy_resources_list);
> > >  	INIT_LIST_HEAD(&vdev->ioeventfds_list);
> > >  	INIT_LIST_HEAD(&vdev->sriov_pfs_item);
> > > +	ret = pcim_p2pdma_init(vdev->pdev);
> > > +	if (ret && ret != -EOPNOTSUPP)
> > > +		return ret;
> >
> > Reading the commit msg seems -EOPNOTSUPP is only returned for fake
> > PCI devices, otherwise it implies regression. better add a comment for it?
> 
> I think the commit log is saying that if a device comes along that
> can't support this, we'd quirk the init path to return -EOPNOTSUPP for
> that particular device here.  This path is currently used when
> !CONFIG_PCI_P2PDMA to make this error non-fatal to the device init.
> 
> I don't see a regression if such a device comes along and while we
> could survive other types of failures by disabling p2pdma here, I think
> all such cases are sufficient rare out of memory cases to consider them
> catastrophic.  Thanks,
> 

ah yes. I read it inaccurately.