linux-kernel - Re: [PATCH 10/35] drm/i915/gvt: Change _PAGE_DIRTY to _PAGE_DIRTY

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Message-ID: <04cbcff2e28e378ece76ab1735a81b945583ad7b.camel@intel.com>
Date:   Fri, 11 Feb 2022 01:39:01 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
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Subject: Re: [PATCH 10/35] drm/i915/gvt: Change _PAGE_DIRTY to
 _PAGE_DIRTY_BITS

CC intel-gfx@...ts.freedesktop.org

Thread: 
https://lore.kernel.org/lkml/a5bb32b8-8bd7-ac98-5c4c-5af604ac8256@intel.com/

On Wed, 2022-02-09 at 08:58 -0800, Dave Hansen wrote:
> On 1/30/22 13:18, Rick Edgecombe wrote:
> > 
> > diff --git a/drivers/gpu/drm/i915/gvt/gtt.c
> > b/drivers/gpu/drm/i915/gvt/gtt.c
> > index 99d1781fa5f0..75ce4e823902 100644
> > --- a/drivers/gpu/drm/i915/gvt/gtt.c
> > +++ b/drivers/gpu/drm/i915/gvt/gtt.c
> > @@ -1210,7 +1210,7 @@ static int split_2MB_gtt_entry(struct
> > intel_vgpu *vgpu,
> >  	}
> >  
> >  	/* Clear dirty field. */
> > -	se->val64 &= ~_PAGE_DIRTY;
> > +	se->val64 &= ~_PAGE_DIRTY_BITS;
> >  
> >  	ops->clear_pse(se);
> >  	ops->clear_ips(se);
> 
> Are these x86 CPU page table values?  I see ->val64 being used like
> this:
> 
>         e->val64 &= ~GEN8_PAGE_PRESENT;
> and
> 	se.val64 |= GEN8_PAGE_PRESENT | GEN8_PAGE_RW;
> 
> where we also have:
> 
> #define GEN8_PAGE_PRESENT               BIT_ULL(0)
> #define GEN8_PAGE_RW                    BIT_ULL(1)
> 
> Which tells me that these are probably *close* to the CPU's page
> tables.
>  But, I honestly don't know which format they are.  I don't know if
> _PAGE_COW is still a software bit in that format or not.
> 
> Either way, I don't think we should be messing with i915 device page
> tables.
> 
> Or, are these somehow magically shared with the CPU in some way I
> don't
> know about?
> 
> [ If these are device-only page tables, it would probably be nice to
>   stop using _PAGE_FOO for them.  It would avoid confusion like this.
> ]

The two Reviewed-by tags are giving me pause, but as far as I can tell
this should not be setting _PAGE_DIRTY_BITS. This code seems to be
shadowing guest page tables, and the change would clear the COW
software bit in the guest page tables. So, yes, I think this should be
dropped.