linux-kernel - Re: [PATCH v2] efi: Fix warning of int-to-pointer-cast on x86 32-bit builds

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Date:	Mon, 26 Oct 2015 21:02:35 +0000
From:	Matt Fleming <matt@...eblueprint.co.uk>
To:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:	Taku Izumi <izumi.taku@...fujitsu.com>,
	Ingo Molnar <mingo.kernel.org@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kamezawa.hiroyu@...fujitsu.com,
	"linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>
Subject: Re: [PATCH v2] efi: Fix warning of int-to-pointer-cast on x86 32-bit
 builds

On Fri, 23 Oct, at 10:37:46AM, Ard Biesheuvel wrote:
> 
> After looking at the original (already merged) patch 11/11 again, I
> realize this is still not right: the problem is that efi_memory_map's
> phys_map member uses a void* type to hold a physical address, which
> happens to be correct in the normal case even when phys_addr_t is
> larger than void* (like on ARM with LPAE enabled) since the address it
> holds is the address of an allocation performed by the firmware, which
> only uses 1:1 addressable memory.
> 
> However, overwriting memmap.phys_map with a value produced my
> memblock_alloc() is problematic, since the allocation may be above 4
> GB on 32-bit (L)PAE platforms. So the correct way to do this would be
> to set the memblock limit to 4GB before memblock_alloc() on 32-bit
> platforms, and restore it afterwards. This is a bit of a kludge,
> though, and it would be more correct to change the type of
> efi_memory_map::phys_map to phys_addr_t, although I don't know what
> the potential fallout of that change is. Matt?

I think that should be fine. The only potentially tricky situation we
could encounter is where 32-bit x86 firmware uses PAE but the kernel
is built without support.

But that's not something I've ever seen enabled in the firmware and
there's a bunch of assumptions in the kernel already that would break
in that case.

Given that addresses are always 64-bit in the UEFI spec, even on
32-bit platforms, what's the downside of picking u64 instead of
phys_addr_t for efi_memory_map::phys_map?
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