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Message-ID: <2ad137cb-ed38-42f6-ac0a-a81569051779@suse.com>
Date: Tue, 30 Sep 2025 17:42:13 +0200
From: Jürgen Groß <jgross@...e.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org, llvm@...ts.linux.dev,
xin@...or.com, "H. Peter Anvin" <hpa@...or.com>,
Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
Nathan Chancellor <nathan@...nel.org>,
Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>
Subject: Re: [PATCH v2 09/12] x86/msr: Use the alternatives mechanism for
WRMSR
On 30.09.25 14:51, Peter Zijlstra wrote:
> On Tue, Sep 30, 2025 at 10:50:44AM +0200, Peter Zijlstra wrote:
>> On Tue, Sep 30, 2025 at 10:46:23AM +0200, Jürgen Groß wrote:
>
>>>> asm_inline volatile goto(
>>>> "1:\n"
>>>> ALTERNATIVE(PREPARE_RCX_RDX_FOR_WRMSR
>>>> "2:\n"
>>>> ALTERNATIVE("ds wrmsr",
>>>> ASM_WRMSRNS, X86_FEATURE_WRMSRNS),
>>>> ASM_WRMSRNS_IMM, X86_FEATURE_MSR_IMM);
>>>> _ASM_EXTABLE_TYPE(1b, %l[badmsr], %c[type]) /* For WRMSRNS immediate */
>>>> _ASM_EXTABLE_TYPE(2b, %l[badmsr], %c[type]) /* For WRMSR(NS) */
>>>>
>>>> :
>>>> : [val] "a" (val), [msr] "i" (msr), [type] "i" (type)
>>>> : "memory", "ecx", "rdx"
>>>> : badmsr);
>>>>
>
>> Oh well, lets forget about this :-)
>
> So I couldn't. I tried the below, which when building a .i generates the
> following:
>
>
> static inline __attribute__((__gnu_inline__)) __attribute__((__unused__)) __attribute__((no_instrument_function)) void clear_page(void *page)
> {
>
> kmsan_unpoison_memory(page, ((1UL) << 12));
> asm __inline volatile(
> "# ALT: oldinstr\n"
> "__UNIQUE_ID_altinstr_9" "_begin:\n\t"
>
> "# ALT: oldinstr\n"
> "__UNIQUE_ID_altinstr_8" "_begin:\n\t"
> "call %c[old]" "\n"
> "__UNIQUE_ID_altinstr_8" "_pad:\n"
> "# ALT: padding\n"
> ".skip -(((" "__UNIQUE_ID_altinstr_8" "_alt_end - " "__UNIQUE_ID_altinstr_8" "_alt_begin" ")-(" "__UNIQUE_ID_altinstr_8" "_pad - " "__UNIQUE_ID_altinstr_8" "_begin" ")) > 0) * "
> "((" "__UNIQUE_ID_altinstr_8" "_alt_end - " "__UNIQUE_ID_altinstr_8" "_alt_begin" ")-(" "__UNIQUE_ID_altinstr_8" "_pad - " "__UNIQUE_ID_altinstr_8" "_begin" ")),0x90\n"
> "__UNIQUE_ID_altinstr_8" "_end:\n"
> ".pushsection .altinstructions,\"a\"\n"
> " .long " "__UNIQUE_ID_altinstr_8" "_begin - .\n"
> " .long " "__UNIQUE_ID_altinstr_8" "_alt_begin - .\n"
> " .4byte " "( 3*32+16)" "\n"
> " .byte " "__UNIQUE_ID_altinstr_8" "_end - " "__UNIQUE_ID_altinstr_8" "_begin" "\n"
> " .byte " "__UNIQUE_ID_altinstr_8" "_alt_end - " "__UNIQUE_ID_altinstr_8" "_alt_begin" "\n"
> ".popsection\n"
> ".pushsection .altinstr_replacement, \"ax\"\n"
> "# ALT: replacement\n"
> "__UNIQUE_ID_altinstr_8" "_alt_begin:\n\t"
> "call %c[new1]" "\n"
> "__UNIQUE_ID_altinstr_8" "_alt_end:\n"
> ".popsection\n"
> "\n"
>
> "__UNIQUE_ID_altinstr_9" "_pad:\n"
> "# ALT: padding\n"
> ".skip -(((" "__UNIQUE_ID_altinstr_9" "_alt_end - " "__UNIQUE_ID_altinstr_9" "_alt_begin" ")-(" "__UNIQUE_ID_altinstr_9" "_pad - " "__UNIQUE_ID_altinstr_9" "_begin" ")) > 0) * "
> "((" "__UNIQUE_ID_altinstr_9" "_alt_end - " "__UNIQUE_ID_altinstr_9" "_alt_begin" ")-(" "__UNIQUE_ID_altinstr_9" "_pad - " "__UNIQUE_ID_altinstr_9" "_begin" ")),0x90\n"
> "__UNIQUE_ID_altinstr_9" "_end:\n"
> ".pushsection .altinstructions,\"a\"\n"
> " .long " "__UNIQUE_ID_altinstr_9" "_begin - .\n"
> " .long " "__UNIQUE_ID_altinstr_9" "_alt_begin - .\n"
> " .4byte " "( 9*32+ 9)" "\n"
> " .byte " "__UNIQUE_ID_altinstr_9" "_end - " "__UNIQUE_ID_altinstr_9" "_begin" "\n"
> " .byte " "__UNIQUE_ID_altinstr_9" "_alt_end - " "__UNIQUE_ID_altinstr_9" "_alt_begin" "\n"
> ".popsection\n"
> ".pushsection .altinstr_replacement, \"ax\"\n"
> "# ALT: replacement\n"
> "__UNIQUE_ID_altinstr_9" "_alt_begin:\n\t"
> "call %c[new2]" "\n"
> "__UNIQUE_ID_altinstr_9" "_alt_end:\n"
> ".popsection\n"
> : "+r" (current_stack_pointer), "=D" (page)
> : [old] "i" (clear_page_orig), [new1] "i" (clear_page_rep), [new2] "i" (clear_page_erms) , "D" (page)
> : "cc", "memory", "rax", "rcx")
>
> ;
> }
>
> Which looks right, but utterly fails to build :(
What does the failure look like?
Could it be that the labels should be local ones?
Juergen
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