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From:   Josh Poimboeuf <jpoimboe@kernel.org>
To:     x86@kernel.org
Cc:     linux-kernel@vger.kernel.org, Borislav Petkov <bp@alien8.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Babu Moger <babu.moger@amd.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <seanjc@google.com>, David.Kaplan@amd.com,
        Andrew Cooper <andrew.cooper3@citrix.com>,
        Nikolay Borisov <nik.borisov@suse.com>,
        gregkh@linuxfoundation.org, Thomas Gleixner <tglx@linutronix.de>
Subject: [PATCH 15/22] x86/alternatives: Remove faulty optimization
Date:   Sun, 20 Aug 2023 18:19:12 -0700
Message-ID: <6820be098b2a450294980d19b0f7b0d75758202d.1692580085.git.jpoimboe@kernel.org>
In-Reply-To: <cover.1692580085.git.jpoimboe@kernel.org>
References: <cover.1692580085.git.jpoimboe@kernel.org>
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The following commit

  095b8303f383 ("x86/alternative: Make custom return thunk

made '__x86_return_thunk' a placeholder value.  All code setting
X86_FEATURE_RETHUNK also changes the value of 'x86_return_thunk'.  So
the optimization at the beginning of apply_returns() is dead code.

Also, before the above-mentioned commit, the optimization actually had a
bug It bypassed __static_call_fixup(), causing some raw returns to
remain unpatched in static call trampolines.  Thus the 'Fixes' tag.

Fixes: d2408e043e72 ("x86/alternative: Optimize returns patching")
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
---
 arch/x86/kernel/alternative.c | 8 --------
 1 file changed, 8 deletions(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 099d58d02a26..34be5fbaf41e 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -720,14 +720,6 @@ void __init_or_module noinline apply_returns(s32 *start, s32 *end)
 {
 	s32 *s;
 
-	/*
-	 * Do not patch out the default return thunks if those needed are the
-	 * ones generated by the compiler.
-	 */
-	if (cpu_feature_enabled(X86_FEATURE_RETHUNK) &&
-	    (x86_return_thunk == __x86_return_thunk))
-		return;
-
 	for (s = start; s < end; s++) {
 		void *dest = NULL, *addr = (void *)s + *s;
 		struct insn insn;
-- 
2.41.0