lkml.org 
[lkml]   [2023]   [Feb]   [13]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [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@suse.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@linux-foundation.org>
>> Signed-off-by: Juergen Gross <jgross@suse.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
[unhandled content-type:application/pgp-keys][unhandled content-type:application/pgp-signature]
\
 
 \ /
  Last update: 2023-03-27 00:19    [W:0.354 / U:0.016 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site