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Message-ID: <Y+I5NyAeu4LWFBcA@zn.tnic>
Date: Tue, 7 Feb 2023 12:42:47 +0100
From: Borislav Petkov <bp@...en8.de>
To: Juergen Gross <jgross@...e.com>, "Kani, Toshi" <toshi.kani@....com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org,
lists@...dbynature.de, mikelley@...rosoft.com,
torvalds@...ux-foundation.org,
Dave Hansen <dave.hansen@...ux.intel.com>,
Andy Lutomirski <luto@...nel.org>,
Peter Zijlstra <peterz@...radead.org>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>,
"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 5/6] x86/mm: only check uniform after calling
mtrr_type_lookup()
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@...ux-foundation.org>
> Signed-off-by: Juergen Gross <jgross@...e.com>
> ---
> 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@zn.tnic
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
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