Re: [PATCH v4 i-g-t] lib/intel_pat: Cache PAT config for unprivileged processes

["Dixit, Ashutosh" <ashutosh.dixit@intel.com>]

On Thu, 05 Mar 2026 12:39:14 -0800, Dixit, Ashutosh wrote:
>
> On Thu, 05 Mar 2026 10:54:31 -0800, <himanshu.girotra@intel.com> wrote:
> >
> > From: Himanshu Girotra <himanshu.girotra@intel.com>
> >
> > Commit 4e59b8e7779b ("lib/intel_pat: use kernel debugfs as
> > authoritative PAT source for Xe") changed the Xe PAT lookup to
> > read from debugfs, which requires root access. Some xe_oa subtests
> > fork a child process that drops root privileges before performing
> > GPU operations that internally need the PAT write-back index.
> > After dropping root, the debugfs read fails.
> >
> > Fix this by caching the PAT configuration in struct xe_device at
> > xe_device_get() time. Since xe_device_get() is called while still
> > root, the cache is available to forked children even after
> > igt_drop_root(). Tests that open a new fd after forking must call
> > xe_device_get() explicitly before dropping root to populate the
> > cache for that fd.
> >
> > v2: Cache PAT in struct xe_device instead of a global static;
> >     drop the intel_pat_precache() helper — xe_device_get() already
> >     populates the cache, so call it directly in xe_oa before
> >     igt_drop_root() (Ashutosh Dixit)
> >
> > v3: Use pat_cache pointer, NULL indicates unpopulated cache;
> >     remove redundant xe_oa.c change (Ashutosh Dixit)
> >
> >     Add FIXME for multi-GPU (Zbigniew Kempczyński)
> >
> > v4: nit fixes and use forward declaration in xe_query.h (Ashutosh Dixit)
>
> Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>

Thanks for the quick turnaround on the fix. The patch is now merged!