Re: [PATCH] reserve end-of-conventional-memory to 1MB on 32-bit v2

Jeremy Fitzhardinge
2008-03-04T17:17:04Z

Alexander van Heukelum wrote:
> On Tue, 04 Mar 2008 07:18:48 -0800, "Jeremy Fitzhardinge"
> <jeremy@goop.org> said:
>
>> Mark McLoughlin wrote:
>>
>>> This is a bit magic, is it worth splitting it out as something like
>>> is_paravirt_environment() ?
>>>
>>>
>> Yes, is_paravirt() already exists for this purpose.
>>
>
> Hi,
>
> If it exists, it is well-hidden. A grep for is_paravirt on the testing
> tree turns up nothing. Did you get the name right?
>

Nope. paravirt_enabled().

>> The code looking at the boot params will only work if we actually booted
>> via the paravirt Linux boot protocol, which Xen doesn't at present.
>>
>
> I chose to code it exactly this way, because it is what is used in
> head_32.S to choose how to start the kernel. Or is this code not
> executed at all?
>

No, this isn't executed when booting under Xen. Xen is a bit magic, and
has its own kernel entrypoint which the domain builder finds via
Xen-specific ELF notes on the vmlinux. We plan to move to booting via
this path at some point.

> [excerpt form head_32.S]
> cmpw $0x207, pa(boot_params + BP_version)
> jb default_entry
>
> /* Paravirt-compatible boot parameters. Look to see what
> architecture
> we're booting under. */
> movl pa(boot_params + BP_hardware_subarch), %eax
> cmpl $num_subarch_entries, %eax
> jae bad_subarch
>
> movl pa(subarch_entries)(,%eax,4), %eax
> subl $__PAGE_OFFSET, %eax
> jmp *%eax
> [and]
> subarch_entries:
> .long default_entry /* normal x86/PC */
> .long lguest_entry /* lguest hypervisor */
> .long xen_entry /* Xen hypervisor */
> num_subarch_entries = (. - subarch_entries) / 4
> [end]
>
> If this is indeed not executed, is there a way to detect whether
> we can expect the environment to behave like a normal pc in terms
> of magic addresses, bios areas, isa reserved address space and so
> on?
>

Just because something is paravirtualized and uses a non-PC
hardware_subarch doesn't mean this stuff isn't present. Even the
paravirt_enabled() test isn't accurate, because the environment may
still choose to emulate these things (or in the Xen dom0 case, it may
directly expose the real hardware).

J