linux-kernel - RE: [PATCH v2 08/22] iommufd: Abstract iopt_pin_pages and iopt_unpin

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Message-ID: <BN9PR11MB5276090879BB4C76CB27909E8C88A@BN9PR11MB5276.namprd11.prod.outlook.com>
Date: Wed, 7 May 2025 07:22:24 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Nicolin Chen <nicolinc@...dia.com>
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Subject: RE: [PATCH v2 08/22] iommufd: Abstract iopt_pin_pages and
 iopt_unpin_pages helpers

> From: Nicolin Chen <nicolinc@...dia.com>
> Sent: Wednesday, May 7, 2025 3:18 AM
> 
> On Tue, May 06, 2025 at 09:36:38AM +0000, Tian, Kevin wrote:
> > > From: Nicolin Chen <nicolinc@...dia.com>
> > > Sent: Saturday, April 26, 2025 1:58 PM
> > >
> > > The new vCMDQ object will be added for HW to access the guest memory
> for
> > > a
> > > HW-accelerated virtualization feature. It needs to ensure the guest
> memory
> > > pages are pinned when HW accesses them and they are contiguous in
> > > physical
> > > address space.
> > >
> > > This is very like the existing iommufd_access_pin_pages() that outputs
> the
> > > pinned page list for the caller to test its contiguity.
> > >
> > > Move those code from iommufd_access_pin/unpin_pages() and related
> > > function
> > > for a pair of iopt helpers that can be shared with the vCMDQ allocator. As
> > > the vCMDQ allocator will be a user-space triggered ioctl function,
> WARN_ON
> > > would not be a good fit in the new iopt_unpin_pages(), thus change them
> to
> > > use WARN_ON_ONCE instead.
> > >
> > > Rename check_area_prot() to align with the existing iopt_area helpers,
> and
> > > inline it to the header since iommufd_access_rw() still uses it.
> > >
> > > Signed-off-by: Nicolin Chen <nicolinc@...dia.com>
> >
> > any reason why this cannot be done by the core? all types of vcmd
> > queues need to pin the guest buffer pages, no matter the IOMMU
> > accesses GPA or HPA.
> >
> > Jason made a similar comment earlier [1].
> >
> > check of continuity is still done by the driver, if HPA is being accessed.
> >
> > [1] https://lore.kernel.org/all/20250424134049.GP1648741@nvidia.com/
> 
> But I am doing in the core. I have iopt_pin_pages() called in the
> core ioctl handler iommufd_vqueue_alloc_ioctl():
> 
> https://lore.kernel.org/linux-
> iommu/1ef2e242ee1d844f823581a5365823d78c67ec6a.1746139811.git.nicoli
> nc@...dia.com/
> 

IMHO we just want to keep the pin logic in the core while leaving
the check of PFN continuity to the driver (or have a way for the 
driver to communicate such need to the core). 

It's possible for have an implementation with IOMMU accessing 
GPA of the queue which further goes through the stage-2 
translation. In such case it's fine to have disjointed PFNs.