Re: [PATCH v3 3/4] x86/alternative: Rewrite optimize_nops() some

[Andrew.Cooper3@citrix.com]

On 08/02/2023 8:29 pm, Peter Zijlstra wrote:
> On Wed, Feb 08, 2023 at 07:52:04PM +0000, Andrew.Cooper3@citrix.com wrote:
>> On 08/02/2023 5:10 pm, Peter Zijlstra wrote:
>>> This rewrite address two issues:
>>>
>>>  - it no longer hard requires single byte nop runs, it now accepts
>>>    any NOP and NOPL encoded instruction (but not the more complicated
>>>    32bit NOPs).
>>>
>>>  - it writes a single 'instruction' replacement.
>>>
>>> Specifically, ORC unwinder relies on the tail NOP of an alternative to
>>> be a single instruction, in particular it relies on the inner bytes
>>> not being executed.
>>>
>>> Once we reach the max supported NOP length (currently 8, could easily
>>> be extended to 11 on x86_64), switches to JMP.d8 and INT3 padding to
>>> achieve the same result.
>>>
>>> The ORC unwinder uses this guarantee in the analysis of
>>> alternative/overlapping CFI state,
>>>
>>> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
>> How lucky are you feeling for your game of performance roulette?
> Yeah, not very lucky.. I've been talking about this with Boris for a bit
> already.
>
>> Unconditional jmps cost branch prediction these days, and won't be
>> successfully predicted until taken.
> IKR, insane, but that's what it is.

In terms of rationalising how things work, sure, put the resulting perf
numbers speak for themselves.

For the benefit of others reading this and not following what's going
on, modern x86 processors have branch prediction occurring pre-decode,
not post-decode, to reduce the misprediction latency.

Branch prediction operates using the current %rip and past history, and
selects the $I lines to send for decode.  The "decoded bytes disagree
with prediction metadata" feedback cycle is fast, but missing this
disagreement is the root cause of the Branch Type Confusion speculation
issue (a.k.a. AMD Retbleed).

~Andrew