linux-kernel - RE: [PATCH 3/8] iommufd: Support attach/replace hwpt per pasid

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Message-ID: <BN9PR11MB52761349DFB5DAD2797C3EBF8C732@BN9PR11MB5276.namprd11.prod.outlook.com>
Date: Tue, 16 Jan 2024 01:18:12 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Jason Gunthorpe <jgg@...dia.com>, "Liu, Yi L" <yi.l.liu@...el.com>
CC: "joro@...tes.org" <joro@...tes.org>, "alex.williamson@...hat.com"
	<alex.williamson@...hat.com>, "robin.murphy@....com" <robin.murphy@....com>,
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 Zhenzhong" <zhenzhong.duan@...el.com>, "joao.m.martins@...cle.com"
	<joao.m.martins@...cle.com>, "Zeng, Xin" <xin.zeng@...el.com>, "Zhao, Yan Y"
	<yan.y.zhao@...el.com>
Subject: RE: [PATCH 3/8] iommufd: Support attach/replace hwpt per pasid

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Tuesday, January 16, 2024 1:25 AM
> 
> On Sun, Nov 26, 2023 at 10:34:23PM -0800, Yi Liu wrote:
> > +/**
> > + * iommufd_device_pasid_detach - Disconnect a {device, pasid} to an
> iommu_domain
> > + * @idev: device to detach
> > + * @pasid: pasid to detach
> > + *
> > + * Undo iommufd_device_pasid_attach(). This disconnects the idev/pasid
> from
> > + * the previously attached pt_id.
> > + */
> > +void iommufd_device_pasid_detach(struct iommufd_device *idev, u32
> pasid)
> > +{
> > +	struct iommufd_hw_pagetable *hwpt;
> > +
> > +	hwpt = xa_load(&idev->pasid_hwpts, pasid);
> > +	if (!hwpt)
> > +		return;
> > +	xa_erase(&idev->pasid_hwpts, pasid);
> > +	iommu_detach_device_pasid(hwpt->domain, idev->dev, pasid);
> > +	iommufd_hw_pagetable_put(idev->ictx, hwpt);
> > +}
> 
> None of this xarray stuff looks locked properly
> 

I had an impression from past discussions that the caller should not
race attach/detach/replace on same device or pasid, otherwise it is
already a problem in a higher level.

and the original intention of the group lock was to ensure all devices
in the group have a same view. Not exactly to guard concurrent
attach/detach.

If this understanding is incorrect we can add a lock for sure. 😊