Re: [PATCH v5 02/10] x86/mm: Avoid hard-coding PAT in get_page_from_l1e()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 20.12.2022 09:19, Jan Beulich wrote:
> On 20.12.2022 02:07, Demi Marie Obenour wrote:
>> get_page_from_l1e() relied on Xen's choice of PAT, which is brittle in
>> the face of future PAT changes.  Instead, compute the actual cacheability
>> used by the CPU and switch on that, as this will work no matter what PAT
>> Xen uses.
>>
>> No functional change intended.  This code is itself questionable and may
>> be removed in the future, but removing it would be an observable
>> behavior change and so is out of scope for this patch series.
>>
>> Signed-off-by: Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>
>> ---
>> Changes since v4:
>> - Do not add new pte_flags_to_cacheability() helper, as this code may be
>>   removed in the near future and so adding a new helper for it is a bad
>>   idea.
>> - Do not BUG() in the event of an unexpected cacheability.  This cannot
>>   happen, but it is simpler to force such types to UC than to prove that
>>   the BUG() is not reachable.
>>
>> Changes since v3:
>> - Compute and use the actual cacheability as seen by the processor.
>>
>> Changes since v2:
>> - Improve commit message.
>> ---
>>  xen/arch/x86/mm.c | 14 ++++++++------
>>  1 file changed, 8 insertions(+), 6 deletions(-)
>>
>> diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
>> index 
>> 78b1972e4170cacccc9c37c6e64e76e66a7da87f..dba6c77ef2f7ed7fcb7f7e526583ccadd35e62cc
>>  100644
>> --- a/xen/arch/x86/mm.c
>> +++ b/xen/arch/x86/mm.c
>> @@ -959,14 +959,16 @@ get_page_from_l1e(
>>              flip = _PAGE_RW;
>>          }
>>  
>> -        switch ( l1f & PAGE_CACHE_ATTRS )
>> +        switch ( 0xFF & (XEN_MSR_PAT >> (8 * pte_flags_to_cacheattr(l1f))) )
>>          {
>> -        case 0: /* WB */
>> -            flip |= _PAGE_PWT | _PAGE_PCD;
>> +        case X86_MT_UC:
>> +        case X86_MT_UCM:
>> +        case X86_MT_WC:
>> +            /* not cacheable */
>>              break;
>> -        case _PAGE_PWT: /* WT */
>> -        case _PAGE_PWT | _PAGE_PAT: /* WP */
>> -            flip |= _PAGE_PCD | (l1f & _PAGE_PAT);
>> +        default:
>> +            /* cacheable */
>> +            flip |= ((l1f & PAGE_CACHE_ATTRS) ^ _PAGE_UC);
>>              break;
> 
> In v4 the comment here was "cacheable, force to UC". The latter aspect is
> quite relevant (and iirc also what Andrew had asked for to have as a
> comment). But with this now being the default case, the comment (in either
> this or the earlier form) would become stale. A forward compatible way of
> wording this would e.g. be "force any other type to UC". With an adjustment
> along these lines (which I think could be done while committing)
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

If you had replied signaling your consent (and perhaps the preferred by you
wording), I would have committed this. Now it's going to be v6 afaic ...

Jan