[PATCH v3 4/4] arm64: prevent __va() translations before memstart_addr is assigned

[Ard Biesheuvel]

On 22 February 2016 at 18:26, Catalin Marinas <catalin.marinas at arm.com> wrote:
> On Mon, Feb 22, 2016 at 04:52:09PM +0000, Will Deacon wrote:
>> On Fri, Feb 12, 2016 at 03:57:26PM +0100, Ard Biesheuvel wrote:
>> > Since memstart_addr is assigned relatively late in the boot code,
>> > after generic code like DT parsing and memblock manipulation has
>> > already occurred, we need to ensure that no __va() translation occur
>> > until memstart_addr has been set to a meaningful value.
>> >
>> > So initialize memstart_addr to a value that cannot represent a valid
>> > physical address, and BUG() if memstart_addr is referenced while it
>> > still holds this value. Note that the > comparison against LLONG_MAX
>> > (not ULLONG_MAX) resolves to a single tbnz instruction that performs
>> > a conditional jump to a brk instruction that is emitted out of line.
>>
>> Even so, I'd imagine that having a measurable impact on system
>> performance. Did you have a go at benchmarking this?
>
> Good point, I forgot about this discussion. We should revert this part
> indeed or at least make it dependent on some config option like
> DEBUG_VM.
>

OK, let me code that up