Re: [PATCH 4/8] efi: Avoid hard-coding the various PAT constants

[Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>]

On Tue, Dec 06, 2022 at 11:17:20AM +0000, Andrew Cooper wrote:
> On 06/12/2022 04:33, Demi Marie Obenour wrote:
> > This makes the code much easier to understand, and avoids problems if
> > Xen's PAT ever changes in the future.
> >
> > Signed-off-by: Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>
> > ---
> >  xen/common/efi/boot.c | 10 +++++-----
> >  1 file changed, 5 insertions(+), 5 deletions(-)
> >
> > diff --git a/xen/common/efi/boot.c b/xen/common/efi/boot.c
> > index 
> > 8e880fe30c7541a202dec3e665300d6549953aa3..260997b251b09dae4b48c1b1db665778e02d760a
> >  100644
> > --- a/xen/common/efi/boot.c
> > +++ b/xen/common/efi/boot.c
> > @@ -1746,21 +1746,21 @@ void __init efi_init_memory(void)
> >          if ( desc->Attribute & EFI_MEMORY_WB )
> >              /* nothing */;
> 
> This is an implicit 0 case which wants changing to _PAGE_WB.

Good catch!  I will make this change in v2, but I also will add
BUILD_BUG_ON(_PAGE_WB), as at least Linux assumes that _PAGE_WB is 0.

> >          else if ( desc->Attribute & EFI_MEMORY_WT )
> > -            prot |= _PAGE_PWT | MAP_SMALL_PAGES;
> > +            prot |= _PAGE_WT | MAP_SMALL_PAGES;
> >          else if ( desc->Attribute & EFI_MEMORY_WC )
> > -            prot |= _PAGE_PAT | MAP_SMALL_PAGES;
> > +            prot |= _PAGE_WC | MAP_SMALL_PAGES;
> >          else if ( desc->Attribute & (EFI_MEMORY_UC | EFI_MEMORY_UCE) )
> > -            prot |= _PAGE_PWT | _PAGE_PCD | MAP_SMALL_PAGES;
> > +            prot |= _PAGE_UC | MAP_SMALL_PAGES;
> >          else if ( efi_bs_revision >= EFI_REVISION(2, 5) &&
> >                    (desc->Attribute & EFI_MEMORY_WP) )
> > -            prot |= _PAGE_PAT | _PAGE_PWT | MAP_SMALL_PAGES;
> > +            prot |= _PAGE_WP | MAP_SMALL_PAGES;
> 
> Unrelated to the transformation. I'm unconvinced about the correctness
> of using MAP_SMALL_PAGES here.  There's nothing wrong with large pages
> of reduced cache-ability, and the framebuffer is going to live in one of
> these regions, probably a WC one...

I can make that a separate patch.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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