Re: [PATCH] efi/arm64: fix fdt-related memory reservation

[Mark Salter <msalter@redhat.com>]

On Mon, 2014-09-08 at 15:06 +0100, Mark Rutland wrote:
> Hi Mark,
> 
> On Mon, Sep 08, 2014 at 02:31:42PM +0100, Mark Salter wrote:
> > Commit 86c8b27a01cf:
> >  "arm64: ignore DT memreserve entries when booting in UEFI mode
> > 
> > prevents early_init_fdt_scan_reserved_mem() from being called for
> > arm64 kernels booting via UEFI. This was done because the kernel
> > will use the UEFI memory map to determine reserved memory regions.
> > That approach has problems in that early_init_fdt_scan_reserved_mem()
> > also reserves the FDT itself and any node-specific reserved memory.
> > By chance of some kernel configs, the FDT may be overwritten before
> > it can be unflattened and the kernel will fail to boot. More subtle
> > problems will result if the FDT has node specific reserved memory
> > which is not really reserved.
> 
> That doesn't sound like fun; apologies for allowing such brokenness
> through in the first place.

Heh. It was obvious that DT unflattening was broken, but bisecting
didn't help much because I kept finding patches which when reverted
made the problem go away even though they obviously weren't the
cause.

> 
> [...]
> 
> > +	/*
> > +	 * Delete all memory reserve map entries. When booting via UEFI,
> > +	 * kernel will use the UEFI memory map to find reserved regions.
> > +	 */
> > +	num_rsv = fdt_num_mem_rsv(fdt);
> > +	for (i = 0; i < num_rsv; i++)
> > +		fdt_del_mem_rsv(fdt, i);
> 
> I don't think that's right. Won't the memreserve entries shift down by
> one each time we call fdt_del_mem_rsv?
> 
> Shouldn't this be something like:
> 
> while (fdt_num_mem_rsv(fdt))
> 	fdt_del_mem_rsv(fdt, 0);
> 
> Or we could count downwards.
> 

Sigh. Yes, you are right. I only tested with one reserved region.
I think counting down would be the way to go. I'll send a fixed
patch shortly.