linux-kernel - [PATCH] x86/msr-index: make SPEC_CTRL

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Date:   Thu,  3 Nov 2022 14:07:48 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Tom Rix <trix@...hat.com>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
        Sandipan Das <sandipan.das@....com>,
        Huang Rui <ray.huang@....com>, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev, kernel-team@...roid.com,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: [PATCH] x86/msr-index: make SPEC_CTRL_IBRS assembler-portable

GNU binutils' assembler (GAS) didn't support L suffixes on immediates
until binutils 2.28 release. Building arch/x86/entry/entry_64.S with GAS
v2.27 will produce the following assembler errors:

  arch/x86/entry/entry_64.S: Assembler messages:
  arch/x86/entry/entry_64.S:308: Error: found 'L', expected: ')'
  arch/x86/entry/entry_64.S:308: Error: found 'L', expected: ')'
  arch/x86/entry/entry_64.S:308: Error: junk `L<<(0)))' after expression
  arch/x86/entry/entry_64.S:596: Error: found 'L', expected: ')'
  arch/x86/entry/entry_64.S:596: Error: found 'L', expected: ')'
  arch/x86/entry/entry_64.S:596: Error: junk `L<<(0)))' after expression

These come from the use of the preprocessor defined SPEC_CTRL_IBRS in
the IBRS_ENTER and IBRS_EXIT assembler macros. SPEC_CTRL_IBRS was using
the BIT macros from include/linux/bits.h which are only portable between
C and assembler for assemblers such as GAS v2.28 (or newer) or clang
because they use the L suffixes for immediate operands, which older GAS
releases cannot parse. The kernel still supports GAS v2.23 and newer
(and older for branches of stable). Let's expand the value of
SPEC_CTRL_IBRS in place so that assemblers don't have issues parsing the
value.

Fixes: 2dbb887e875b ("x86/entry: Add kernel IBRS implementation")
Reported-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
---
Some other ideas considered:
* Use U64_C from include/asm-generic/int-ll64.h rather than BIT for the
  value of SPEC_CTRL_IBRS.
  * Do so for the entirety of arch/x86/include/asm/msr-index.h or just
    SPEC_CTRL_IBRS? include/asm-generic/int-ll64.h doesn't define a UL
    suffix; add one?
* Make include/linux/bits.h assembler-portable (for older assemblers)
  via the use of include/asm-generic/int-ll64.h.

 arch/x86/include/asm/msr-index.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
index 10ac52705892..0192d853136c 100644
--- a/arch/x86/include/asm/msr-index.h
+++ b/arch/x86/include/asm/msr-index.h
@@ -46,7 +46,7 @@
 #define MSR_TEST_CTRL_SPLIT_LOCK_DETECT		BIT(MSR_TEST_CTRL_SPLIT_LOCK_DETECT_BIT)
 
 #define MSR_IA32_SPEC_CTRL		0x00000048 /* Speculation Control */
-#define SPEC_CTRL_IBRS			BIT(0)	   /* Indirect Branch Restricted Speculation */
+#define SPEC_CTRL_IBRS			1	   /* Indirect Branch Restricted Speculation */
 #define SPEC_CTRL_STIBP_SHIFT		1	   /* Single Thread Indirect Branch Predictor (STIBP) bit */
 #define SPEC_CTRL_STIBP			BIT(SPEC_CTRL_STIBP_SHIFT)	/* STIBP mask */
 #define SPEC_CTRL_SSBD_SHIFT		2	   /* Speculative Store Bypass Disable bit */
-- 
2.38.1.431.g37b22c650d-goog