Re: [PATCH 5/6] x86/mm: only check uniform after calling mtrr_type_lookup()

[Juergen Gross]

On 07.02.23 12:42, Borislav Petkov wrote:
> On Tue, Feb 07, 2023 at 08:29:01AM +0100, Juergen Gross wrote:
> > Today pud_set_huge() and pmd_set_huge() test for the MTRR type to be
> > WB or INVALID after calling mtrr_type_lookup(). Those tests can be
> > dropped, as the only reason to not use a large mapping would be
> > uniform being 0. Any MTRR type can be accepted as long as it applies
> > to the whole memory range covered by the mapping, as the alternative
> > would only be to map the same region with smaller pages instead using
> > the same PAT type as for the large mapping.
> >
> > Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> > ---
> >   arch/x86/mm/pgtable.c | 6 ++----
> >   1 file changed, 2 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
> > index e4f499eb0f29..7b9c5443d176 100644
> > --- a/arch/x86/mm/pgtable.c
> > +++ b/arch/x86/mm/pgtable.c
> > @@ -721,8 +721,7 @@ int pud_set_huge(pud_t *pud, phys_addr_t addr, pgprot_t prot)
> >   	u8 mtrr, uniform;
> >   
> >   	mtrr = mtrr_type_lookup(addr, addr + PUD_SIZE, &uniform);
> > -	if ((mtrr != MTRR_TYPE_INVALID) && (!uniform) &&
> > -	    (mtrr != MTRR_TYPE_WRBACK))
> > +	if (!uniform)
> >   		return 0;
> >   
> >   	/* Bail out if we are we on a populated non-leaf entry: */
> > @@ -748,8 +747,7 @@ int pmd_set_huge(pmd_t *pmd, phys_addr_t addr, pgprot_t prot)
> >   	u8 mtrr, uniform;
> >   
> >   	mtrr = mtrr_type_lookup(addr, addr + PMD_SIZE, &uniform);
> > -	if ((mtrr != MTRR_TYPE_INVALID) && (!uniform) &&
> > -	    (mtrr != MTRR_TYPE_WRBACK)) {
> > +	if (!uniform) {
> >   		pr_warn_once("%s: Cannot satisfy [mem %#010llx-%#010llx] with a huge-page mapping due to MTRR override.\n",
> >   			     __func__, addr, addr + PMD_SIZE);
> >   		return 0;
> > --
>
> See my reply here:
>
> https://lore.kernel.org/all/Y+DLqV5MfuBJRnb6@xxxxxxx
>
> I understand it as WB is ok, for example, even if not uniform. That
> thing in mtrr_type_lookup():
>
>          /*
>           * Look up the fixed ranges first, which take priority over
>           * the variable ranges.
>           */
>          if ((start < 0x100000) &&
>              (mtrr_state.have_fixed) &&
>              (mtrr_state.enabled & MTRR_STATE_MTRR_FIXED_ENABLED)) {
>                  is_uniform = 0;
>                  type = mtrr_type_lookup_fixed(start, end);
>                  goto out;
>          }
>
> If that can return WB, then I guess that says it is still ok. Can the
> fixed ranges even cover a, at least a PMD? I guess I need to stare at
> this more.

Fixed MTRRs are all below 1MB. So no, they can't cover a PMD.


Juergen

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