Re: [Xen-devel] [PATCH v2 3/8] xen/x86: manually build xen.mb.efi binary

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 10.07.18 at 12:48, <daniel.kiper@xxxxxxxxxx> wrote:
> On Fri, Jul 06, 2018 at 09:08:29AM -0600, Jan Beulich wrote:
>> >>> On 06.07.18 at 16:02, <daniel.kiper@xxxxxxxxxx> wrote:
>> > On Thu, Jul 05, 2018 at 02:18:03AM -0600, Jan Beulich wrote:
>> >> >>> On 04.07.18 at 18:35, <daniel.kiper@xxxxxxxxxx> wrote:
>> >> > On Wed, Jul 04, 2018 at 09:27:43AM -0600, Jan Beulich wrote:
>> >> >> >>> On 04.07.18 at 16:01, <daniel.kiper@xxxxxxxxxx> wrote:
>> >> >> > On Mon, Jun 25, 2018 at 09:36:07AM -0600, Jan Beulich wrote:
>> >> >> >> >>> On 19.06.18 at 16:35, <daniel.kiper@xxxxxxxxxx> wrote:
>> >> >> >> > @@ -582,6 +587,12 @@ static void __init 
>> >> >> >> > efi_arch_memory_setup(void)
>> >> >> >> >      if ( !efi_enabled(EFI_LOADER) )
>> >> >> >> >          return;
>> >> >> >> >
>> >> >> >> > +    if ( efi_enabled(EFI_MB_LOADER) )
>> >> >> >> > +        for ( pte = __page_tables_start; pte < __page_tables_end;
>> >> >> >> > +              pte += ( pte != (intpte_t *)l2_identmap ) ? 1 : 4 
>> >> >> >> > * L2_PAGETABLE_ENTRIES )
>> >> >> >>
>> >> >> >> Please avoid explicit casts - &l2e_get_intpte(l2_identmap[0]) or
>> >> >> >> something along those lines ought to work here. Same for
>> >> >> >> 4 * L2_PAGETABLE_ENTRIES - you mean ARRAY_SIZE() there.
>> >> >> >
>> >> >> > OK.
>> >> >> >
>> >> >> >> Also this whole code block needs a comment, to explain what it
>> >> >> >> does and also why l2_identmap needs skipping.
>> >> >> >>
>> >> >> >> Furthermore - isn't this off by one, and you process l2_identmap[0]
>> >> >> >> this way, skipping the rest _plus_ the first following entry? I 
>> >> >> >> think
>> >> >> >
>> >> >> > The code mimics similar code in head.S.
>> >> >>
>> >> >> I can't see a similar off-by-1 in head.S.
>> >> >
>> >> >  662         /*
>> >> >  663          * Update frame addresses in page tables excluding 
>> >> > l2_identmap
>> >> >  664          * without its first entry which points to l1_identmap.
>> >> >  665          */
>> >> >  666         mov     $((__page_tables_end-__page_tables_start)/8),%ecx
>> >> >  667         mov     $(((l2_identmap-__page_tables_start)/8)+1),%edx
>> >> >  668 1:      cmp     
>> >> > $((l2_identmap+l2_identmap_sizeof-__page_tables_start)/8),%ecx
>> >> >  669         cmove   %edx,%ecx
>> >> >  670         testl   
>> >> > $_PAGE_PRESENT,sym_fs(__page_tables_start)-8(,%ecx,8)
>> >> >  671         jz      2f
>> >> >  672         add     %esi,sym_fs(__page_tables_start)-8(,%ecx,8)
>> >> >  673 2:      loop    1b
>> >>
>> >> Well - this is the code in question, but you fail to point out where
>> >> the off-by-1 is.
>> >
>> > Line 667, 668 and 669.
>>
>> I don't think so, no. Note the -8 in lines 670 and 672.
> 
> However, you are missing +1 in line 667.

I don't think I am: The first entry of l2_identmap actually needs
processing afaics (and as the comment says), as that's the only one
with non-absolute contents. IOW - part of my original comment was
wrong, but the other half (you skipping one entry) still seems
applicable, as does the part concerning the missing comment.

Jan



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