Re: Kernel broken on processors without performance counters

[Jason Baron <jasonbaron0@gmail.com>]

On 07/23/2015 01:08 PM, Peter Zijlstra wrote:
> On Thu, Jul 23, 2015 at 11:34:50AM -0400, Steven Rostedt wrote:
>> On Thu, 23 Jul 2015 12:42:15 +0200
>> Peter Zijlstra <peterz@infradead.org> wrote:
>>
>>> static __always_inline bool arch_static_branch_jump(struct static_key *key, bool inv)
>>> {
>>> 	if (!inv) {
>>> 		asm_volatile_goto("1:"
>>> 			"jmp %l[l_yes]\n\t"
>> And what happens when this gets converted to a two byte jump?
>>
> That would be bad, how can we force it to emit 5 bytes?
hmm....I don't think that's an issue, the patching code can
detect if its a 2-byte jump - 0xeb, or 5-byte: 0xe9, and do
the correct no-op. Same going the other way. See the code
I posted a few mails back. In fact, this gets us to the
smaller 2-byte no-ops in cases where we are initialized
to jump.

Thanks,

-Jason