Re: [PATCH] efi: get_memory_map: add sufficient slack for memory descriptors

[Mark Rutland <mark.rutland-5wv7dgnIgG8@public.gmane.org>]

On Thu, Feb 12, 2015 at 02:56:51PM +0000, Ard Biesheuvel wrote:
> On 12 February 2015 at 22:47, Matt Fleming <matt-mF/unelCI9GS6iBeEJttW/XRex20P6io@public.gmane.org> wrote:
> > On Thu, 12 Feb, at 06:39:46PM, Ard Biesheuvel wrote:
> >>
> >> I don't see how doing a single allocation could result in a single
> >> free region to be split into more than 1 occupied region + 2 free
> >> regions.
> >> So no, I don't think it is ...
> >
> > I don't think that's a guarantee we can make, nor is it something we
> > should rely upon.
> >
> > Please explain the user-visible failure that this patch fixes. Does your
> > machine refuse to boot?
> 
> I am running UEFI under QEMU and Xen primarily at the moment, and
> experimenting with various build options in Tianocore, One of the
> options is preallocating and freeing blocks of various memory types,
> in a way that should result in the final number of distinct regions to
> be much lower. It could result however in a free memory region to be
> carved up in three instead of two, and that is a failure I have seen
> occur.

The simple answer is that the machine will fail to boot, beause the
efi_get_memory_map helper will give up after one go, and propagate the
error. The arm-stub will give up when the error is encountered.

> > Why is the 'goto again' loop insufficient in
> > handling this scenario?
> >
> 
> Yes, that should solve it as well, so if you prefer I reinstate that,
> I can respin the patch. There is a theoretical possibility that it
> would take more than just one more iteration, but that is highly
> unlikely and it should still always complete.

Please reinstate the loop. It will make this far less fragile.

Thanks,
Mark.