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

[Ard Biesheuvel]

On 22 February 2016 at 17:52, Will Deacon <will.deacon at arm.com> 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?
>

So in what kind of workload would the __pa() translation be on a hot
path? If you're dealing with DMA or other things that involve physical
addresses, surely, the single predicted non-taken branch instruction
shouldn't hurt?