Re: [Xen-devel] [PATCH] x86: fix comment on super page alignment requirement

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 24.09.18 at 15:39, <wei.liu2@xxxxxxxxxx> wrote:
> On Mon, Sep 24, 2018 at 05:19:12AM -0600, Jan Beulich wrote:
>> >>> On 24.09.18 at 12:38, <wei.liu2@xxxxxxxxxx> wrote:
>> > --- a/xen/arch/x86/setup.c
>> > +++ b/xen/arch/x86/setup.c
>> > @@ -944,12 +944,13 @@ void __init noreturn __start_xen(unsigned long mbi_p)
>> >  
>> >      /*
>> >       * Iterate backwards over all superpage-aligned RAM regions.
>> > -     * 
>> > -     * We require superpage alignment because the boot allocator is not 
>> > yet
>> > -     * initialised. Hence we can only map superpages in the address range
>> > -     * 0 to BOOTSTRAP_DIRECTMAP_END, as this is guaranteed not to require
>> > +     *
>> > +     * We require superpage alignment because the boot allocator is
>> > +     * not yet initialised. Hence we can only map superpages in the
>> > +     * address range BOOTSTRAP_MAP_BASE to (BOOTSTRAP_MAP_BASE +
>> > +     * BOOTSTRAP_MAP_LIMIT), as this is guaranteed not to require
>> 
>> The upper bound is not a sum. But there's then also an apparent
>> disconnect: BOOTSTRAP_MAP_LIMIT !=
>> (ARRAY_SIZE(l2_identmap) << L2_PAGETABLE_SHIFT) afaict, yet
>> the latter is what is used for mapping (and what matches the value
>> of BOOTSTRAP_DIRECTMAP_END in 4.0.4).
> 
> Yeah, I got a few surprises while reading that code. I'm trying to
> figure out what the limit should be at the moment.

4Gb

Jan



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