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

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Date: Wed, 17 Jan 2024 04:17:39 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Jason Gunthorpe <jgg@...dia.com>
CC: "Liu, Yi L" <yi.l.liu@...el.com>, "joro@...tes.org" <joro@...tes.org>,
	"alex.williamson@...hat.com" <alex.williamson@...hat.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 8:58 PM
> 
> On Tue, Jan 16, 2024 at 01:18:12AM +0000, Tian, Kevin wrote:
> > > 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.
> 
> I thought that was just at the iommu layer? We want VFIO to do the
> same? Then why do we need the dual xarrays?
> 
> Still, it looks really wrong to have code like this, we don't need to
> - it can be locked properly, it isn't a performance path..

OK, let's add a lock for this.

> 
> > 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.
> 
> We don't have a group lock here, this is in iommufd.

I meant the lock in iommufd_group.

> 
> Use the xarray lock..
> 
> eg
> 
> hwpt = xa_erase(&idev->pasid_hwpts, pasid);
> if (WARN_ON(!hwpt))
>    return
> 
> xa_erase is atomic.
> 

yes, that's better.