From: Andrew Cooper <andrew.cooper3@citrix.com>
To: "H. Peter Anvin" <hpa@zytor.com>, Peter Zijlstra <peterz@infradead.org>
Cc: x86@kernel.org, kees@kernel.org, alyssa.milburn@intel.com,
scott.d.constable@intel.com, joao@overdrivepizza.com,
samitolvanen@google.com, nathan@kernel.org,
alexei.starovoitov@gmail.com, mhiramat@kernel.org,
ojeda@kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [RFC][PATCH] x86,ibt: Use UDB instead of 0xEA
Date: Fri, 15 Aug 2025 18:21:48 +0100 [thread overview]
Message-ID: <7594ecd7-994d-4cb8-9f8a-b6c1d4b01e17@citrix.com> (raw)
In-Reply-To: <78bd985a-a59e-4469-a84d-a0eb7faa20c3@zytor.com>
On 15/08/2025 6:14 pm, H. Peter Anvin wrote:
> On 2025-08-15 04:19, Andrew Cooper wrote:
>>> CS Jcc, decodes to Jcc,pn for non-taken
>>> DS Jcc, decodes to Jcc,pt for taken
>> Ah, thanks. I was looking at the hex in one of the comments and still
>> couldn't figure it out.
>>
>> So with this notation, we also have the dual meaning of ,pt between the
>> P4 and LNC. At least the encoding is the same.
>>
> What "dual meaning?"
Well, it has different semantics now it's been reintroduced in LNC.
(Only has any effect on a prediction miss, and causes a proactive decode
resteer).
Sure, it's "just perf" so can be argued as "compatible behaviour", but
people caring about the ,pt / ,pn properties do need to know the different.
~Andrew
next prev parent reply other threads:[~2025-08-15 17:21 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-08-14 11:17 [RFC][PATCH] x86,ibt: Use UDB instead of 0xEA Peter Zijlstra
2025-08-14 11:23 ` Peter Zijlstra
2025-08-14 19:40 ` Nathan Chancellor
2025-08-15 1:27 ` H. Peter Anvin
2025-08-15 7:49 ` Peter Zijlstra
2025-08-15 10:28 ` Peter Zijlstra
2025-08-15 10:30 ` Peter Zijlstra
2025-08-15 10:43 ` Andrew Cooper
2025-08-15 10:59 ` Peter Zijlstra
2025-08-15 11:19 ` Andrew Cooper
2025-08-15 17:14 ` H. Peter Anvin
2025-08-15 17:21 ` Andrew Cooper [this message]
2025-08-15 17:28 ` H. Peter Anvin
2025-08-15 5:42 ` Alexei Starovoitov
2025-08-15 7:57 ` Peter Zijlstra
2025-08-15 8:28 ` Leon Hwang
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