linux-kernel - Re: [PATCH v2 7/8] x86/mm: only check uniform after calling mtrr_type

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Date:   Mon, 13 Feb 2023 07:35:29 +0100
From:   Juergen Gross <jgross@...e.com>
To:     "Michael Kelley (LINUX)" <mikelley@...rosoft.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>
Cc:     "lists@...dbynature.de" <lists@...dbynature.de>,
        "torvalds@...ux-foundation.org" <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>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v2 7/8] x86/mm: only check uniform after calling
 mtrr_type_lookup()

On 13.02.23 02:08, Michael Kelley (LINUX) wrote:
> From: Juergen Gross <jgross@...e.com> Sent: Wednesday, February 8, 2023 11:22 PM
>>
>> 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);
> 
> I'm seeing this warning trigger in a normal Hyper-V guest (i.e., *not* an
> SEV-SNP Confidential VM).  The original filtering here based on
> MTRR_TYPE_WRBACK appears to be hiding a bug in mtrr_type_lookup_variable()
> where it incorrectly thinks an address range matches two different variable
> MTRRs, and hence clears "uniform".
> 
> Here are the variable MTRRs in the normal Hyper-V guest with 32 GiBytes
> of memory:
> 
> [    0.043592] MTRR variable ranges enabled:
> [    0.048308]   0 base 000000000000 mask FFFF00000000 write-back
> [    0.057450]   1 base 000100000000 mask FFF000000000 write-back
> [    0.063972]   2 disabled
> [    0.066755]   3 disabled
> [    0.070024]   4 disabled
> [    0.072856]   5 disabled
> [    0.076112]   6 disabled
> [    0.078760]   7 disabled
> 
> Variable MTRR #0 covers addresses up to 4 GiByte, while #1 covers
> 4 GiByte to 64 GiByte.   But in mtrr_type_lookup_variable(), address
> range 0xF8000000 to 0xF81FFFFF is matching both MTRRs, when it
> should be matching just #0.
> 
> The problem looks to be this code in mtrr_type_lookup_variable():
> 
>                if ((start & mask) != (base & mask))
>                          continue;
> 
> If the mask bits of start and base are different, then the
> MTRR doesn't match, and the continue statement should be
> executed.  That's correct.  But if the mask bits are the same,
> that's not sufficient for the MTRR to match.  If the end
> address is less than base, the MTRR doesn't match, and
> the continue statement should still be executed, which
> isn't happening.
> 
> But somebody please check my thinking. :-)

I don't see a flaw in your reasoning.

Rick mentioned a problem with this patch in a KVM guest. I'll try to
reproduce his setup for checking whether fixing mtrr_type_lookup_variable()
is enough, or if we need to keep the tests for WB in this patch.


Juergen

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