Re: [PATCH 1/3] x86, mm: Include the entire kernel memory map in trampoline_pgd

["Jan Beulich" <JBeulich@suse.com>]

>>> On 05.10.12 at 08:48, Matt Fleming <matt@console-pimps.org> wrote:
> On Fri, 2012-10-05 at 07:39 +0100, Jan Beulich wrote:
>> >>> On 04.10.12 at 23:08, "H. Peter Anvin" <hpa@zytor.com> wrote:
>> > On 10/03/2012 06:31 AM, Jan Beulich wrote:
>> >>>>> Matt Fleming <matt@console-pimps.org> 10/03/12 2:59 PM >>>
>> >>> +static int insert_identity_mapping(resource_size_t paddr, unsigned long 
>> > vaddr,
>> >>> +                    unsigned long size)
>> >>> +{
>> >>> +    unsigned long end = vaddr + size;
>> >>> +    unsigned long next;
>> >>> +    pgd_t *vpgd, *ppgd;
>> >>> +
>> >>> +#ifdef CONFIG_X86_32
>> >>> +    ppgd = initial_page_table + pgd_index(paddr);
>> >>> +
>> >>> +    if (paddr >= PAGE_OFFSET || paddr + size > PAGE_OFFSET)
>> >>> +        return 1;
>> >>> +#else
>> >>> +    ppgd = __va(real_mode_header->trampoline_pgd) + pgd_index(paddr);
>> >>
>> >> Missing equivalent code (to the 32-bit one above) here - after all, you're 
>> > trying
>> >> to potentially insert a 52-bit physical address into 48-bit virtual space.
>> >>
>> > 
>> > We should have the check, but at least for Linux support we require
>> > P <= V-2.
>> 
>> Not really imo - P <= V - 1 should be sufficient here, as all that is
>> necessary is that the result represents a 1:1 mapping. Specifically,
>> there's no constraint to the virtual space limitation of the direct
>> mapping of RAM.
> 
> Just to be clear, I was going to add this check,
> 
>         /* Don't map over the guard hole. */
>         if (paddr >= 0x7fffffffffff || paddr + size > 0x7fffffffffff)
>                 return 1;

0x800000000000 would be the right numbers in both cases.

> Since I'm guessing mapping over the guard hole would be bad. 

Really, you just can't map anything there (you'd most likely end
up mapping something at 0xffff800000000000, and that would
indeed be bad).

Jan