linux-kernel - Re: [PATCH 5/6] x86/mm: only check uniform after calling mtrr_type

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Date:   Tue, 7 Feb 2023 12:54:53 +0100
From:   Juergen Gross <jgross@...e.com>
To:     Borislav Petkov <bp@...en8.de>, "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 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@...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.

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


Juergen

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