Re: [PATCH v2 2/4] x86/mm: special case super page alignment detection for INVALID_MFN

[Jan Beulich <jbeulich@xxxxxxxx>]

On 07.11.2024 16:52, Roger Pau Monné wrote:
> On Thu, Nov 07, 2024 at 12:06:41PM +0100, Jan Beulich wrote:
>> On 06.11.2024 13:29, Roger Pau Monne wrote:
>>> --- a/xen/arch/x86/include/asm/page.h
>>> +++ b/xen/arch/x86/include/asm/page.h
>>> @@ -202,7 +202,8 @@ static inline l4_pgentry_t l4e_from_paddr(paddr_t pa, 
>>> unsigned int flags)
>>>  
>>>  /* Check if an address is aligned for a given slot level. */
>>>  #define SLOT_IS_ALIGNED(v, m, s) \
>>> -    IS_ALIGNED(PFN_DOWN(v) | mfn_x(m), (1UL << ((s) - PAGE_SHIFT)) - 1)
>>> +    IS_ALIGNED(PFN_DOWN(v) | (mfn_eq(m, INVALID_MFN) ? 0 : mfn_x(m)), \
>>> +               (1UL << ((s) - PAGE_SHIFT)) - 1)
>>>  #define IS_L3E_ALIGNED(v, m) SLOT_IS_ALIGNED(v, m, L3_PAGETABLE_SHIFT)
>>>  #define IS_L2E_ALIGNED(v, m) SLOT_IS_ALIGNED(v, m, L2_PAGETABLE_SHIFT)
>>
>> With this adjustment it feels yet more important for these macros to
>> become local ones in x86/mm.c. This special property may not be what one
>> wants in the general case. And m is now also evaluated twice (really:
>> once or twice), which a "random" user of the macro may not like.
>>
>> I'm further uncertain now that this is the way to go to address the
>> original issue. Notably for the 1G-mapping case it may be better to go
>> from the incoming flags having _PAGE_PRESENT clear. After all we can
>> always create non-present "large" PTEs. E.g.
> 
> Hm, I don't think we want to do that in map_pages_to_xen() as part of
> this change.  Doing so would possibly imply the freeing of
> intermediate page-tables when Xen previously didn't free them.  If the
> CPU didn't support 1GB mappings we would always keep the L2, even if
> fully empty.  With your proposed change we would now free such L2.
> 
> I'm not saying it's a wrong change, but just didn't want to put this
> extra change of behavior together with a bugfix for an existing issue.

I can understand your concern here; perhaps indeed best to keep that
adjustment separate.

>>         if ( (cpu_has_page1gb || !(flags & _PAGE_PRESENT)) &&
>>              IS_L3E_ALIGNED(virt, flags & _PAGE_PRESENT ? mfn : _mfn(0)) &&
>>              nr_mfns >= (1UL << (L3_PAGETABLE_SHIFT - PAGE_SHIFT)) &&
>>              !(flags & (_PAGE_PAT | MAP_SMALL_PAGES)) )
>>
>> Thoughts?
> 
> I was doing it based on mfn because that's how it worked previously
> when 0 was passed instead of INVALID_MFN, and because I think it was
> cleaner to hide the evaluation inside of IS_LnE_ALIGNED() instead of
> open-coding it for every call to IS_LnE_ALIGNED().
> 
> If we want to do it based on flags it would be best if those are
> passed to IS_LnE_ALIGNED(), but again, might be best to do it in a
> followup patch and not part of this bugfix.  I fear it could have
> unpredicted consequences.

Here, however, I view the flags-based approach as simply a different
(and imo more correct) way of addressing the issue at hand. The special
casing of MFN 0 had always been somewhat bogus imo, just that in the
old days we didn't even have a proper INVALID_MFN.

Jan