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

[Borislav Petkov]

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.

Lemme add Toshi who authored that code - he might have a comment or two.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette