Re: [tip:efi/core] x86/mm/pat: Use _PAGE_GLOBAL bit for EFI page table mappings

[Sai Praneeth Prakhya]

On Tue, 2016-02-23 at 09:47 -0800, Andy Lutomirski wrote:
> On Feb 23, 2016 1:09 AM, <&quot;tip-bot for Sai Praneeth
> &lt;tipbot@xxxxxxxxx&gt;&quot;@zytor.com> wrote:
> 
> Something's wrong with tip-bot.  This should say:
> 
> 
> commit 397630150632639b3ca5b4414accd5011c45e276
> Author: Sai Praneeth <sai.praneeth.prakhya@xxxxxxxxx>
> Date:   Wed Feb 17 12:35:56 2016 +0000
> 
>     x86/mm/pat: Use _PAGE_GLOBAL bit for EFI page table mappings
> 
>     Since EFI page tables can be treated as kernel page tables they should
>     be global. All the other page mapping functions in pageattr.c set the
>     _PAGE_GLOBAL bit and we want to avoid inconsistencies when we map a page
>     in the EFI code paths, for example when that page is split in
>     __split_large_page(), etc. It also makes it easier to validate that the
>     EFI region mappings have the correct attributes because there are fewer
>     differences compared with regular kernel mappings.
> 
> But the actual patch is:
> 
> 
> @@ -909,6 +909,20 @@ static void populate_pte(struct cpa_data *cpa,
> 
>         pte = pte_offset_kernel(pmd, start);
> 
> +       /*
> +        * Set the GLOBAL flags only if the PRESENT flag is
> +        * set otherwise pte_present will return true even on
> +        * a non present pte. The canon_pgprot will clear
> +        * _PAGE_GLOBAL for the ancient hardware that doesn't
> +        * support it.
> +        */
> +       if (pgprot_val(pgprot) & _PAGE_PRESENT)
> +               pgprot_val(pgprot) |= _PAGE_GLOBAL;
> +       else
> +               pgprot_val(pgprot) &= ~_PAGE_GLOBAL;
> +
> +       pgprot = canon_pgprot(pgprot);
> +
> 
> The comment is confusing.  This code is setting GLOBAL if PRESENT is
> set even if not requested, but the comment is about setting GLOBAL
> *only* if PRESENT is set.

As you rightly said the code is about making a page GLOBAL if PRESENT is
set and we do set PRESENT bit before mapping so that page is GLOBAL.
This code was taken from the other parts of pageattr.c. The point is
that we don't want differences between whether things were mapped in the
EFI page tables directly (i.e. using populate_pte()) or later split from
large pages via the split_large_page() code path. If this is still
confusing could you please elaborate on it further.

> 
> Can you explain:
> 
> a) Why this wasn't already broken. (were there no callers who set
> GLOBAL but not PRESENT?  If there weren't any, why is that part
> needed?)
> 

I don't think previous implementation is broken and this is not a bug
fix as such. Before this patch some EFI region mappings had GLOBAL bit
set (which followed split large page path) and some aren't (which used
populate_pte). This patch just aligns the mappings done via two
different code paths as mentioned above. As a whole it also maintains
consistency with kernel mappings.

> b) Why setting GLOBAL for EFI mappings is useful.

We did this for consistency among EFI mappings. This has some advantages
as mentioned in the commit message. It also makes it less confusing when
starting at the PGT_DUMP traces if _PAGE_GLOBAL is used consistently.

We don't actually do anything special with _PAGE_GLOBAL in EFI.

> c) Why setting GLOBAL for EFI mappings is safe.  Don't we unmap the
> EFI mappings when we're not actively using them in new kernels?  If
> so, don't we explicitly want them *not* to be GLOBAL to avoid needing
> an extra-expensive global flush?

This is a valid point. I know that EFI runtime regions persist during
and after boot if we have a UEFI firmware and other commits made EFI
regions have separate page table but I am not clear about the effect of
global flush. I think Matt/Boris could comment on it.

> d) Why this doesn't break any non-EFI code.

We touch this code path only when mapping EFI runtime regions to VA
space, i.e. we added pgd field in cpa only as a support for mapping efi
runtime regions.

> --Andy