From: Peter Zijlstra <peterz@infradead.org>
To: "Jürgen Groß" <jgross@suse.com>
Cc: linux-kernel@vger.kernel.org, x86@kernel.org,
llvm@lists.linux.dev, xin@zytor.com,
"H. Peter Anvin" <hpa@zytor.com>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
Dave Hansen <dave.hansen@linux.intel.com>,
Nathan Chancellor <nathan@kernel.org>,
Nick Desaulniers <nick.desaulniers+lkml@gmail.com>,
Bill Wendling <morbo@google.com>,
Justin Stitt <justinstitt@google.com>
Subject: Re: [PATCH v2 09/12] x86/msr: Use the alternatives mechanism for WRMSR
Date: Tue, 30 Sep 2025 14:51:56 +0200 [thread overview]
Message-ID: <20250930125156.GK1386988@noisy.programming.kicks-ass.net> (raw)
In-Reply-To: <20250930085044.GK3245006@noisy.programming.kicks-ass.net>
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 :(
diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h
index 15bc07a5ebb3..12a93982457a 100644
--- a/arch/x86/include/asm/alternative.h
+++ b/arch/x86/include/asm/alternative.h
@@ -5,6 +5,7 @@
#include <linux/types.h>
#include <linux/stringify.h>
#include <linux/objtool.h>
+#include <linux/compiler.h>
#include <asm/asm.h>
#include <asm/bug.h>
@@ -184,38 +185,41 @@ static inline int alternatives_text_reserved(void *start, void *end)
#define ALT_CALL_INSTR "call BUG_func"
-#define alt_slen "772b-771b"
-#define alt_total_slen "773b-771b"
-#define alt_rlen "775f-774f"
+#define alt_slen(pfx) #pfx "_pad - " #pfx "_begin"
+#define alt_total_slen(pfx) #pfx "_end - " #pfx "_begin"
+#define alt_rlen(pfx) #pfx "_alt_end - " #pfx "_alt_begin"
-#define OLDINSTR(oldinstr) \
+#define OLDINSTR(oldinstr, pfx) \
"# ALT: oldinstr\n" \
- "771:\n\t" oldinstr "\n772:\n" \
+ #pfx "_begin:\n\t" oldinstr "\n" #pfx "_pad:\n" \
"# ALT: padding\n" \
- ".skip -(((" alt_rlen ")-(" alt_slen ")) > 0) * " \
- "((" alt_rlen ")-(" alt_slen ")),0x90\n" \
- "773:\n"
+ ".skip -(((" alt_rlen(pfx) ")-(" alt_slen(pfx) ")) > 0) * " \
+ "((" alt_rlen(pfx) ")-(" alt_slen(pfx) ")),0x90\n" \
+ #pfx "_end:\n"
-#define ALTINSTR_ENTRY(ft_flags) \
+#define ALTINSTR_ENTRY(ft_flags, pfx) \
".pushsection .altinstructions,\"a\"\n" \
- " .long 771b - .\n" /* label */ \
- " .long 774f - .\n" /* new instruction */ \
+ " .long " #pfx "_begin - .\n" /* label */ \
+ " .long " #pfx "_alt_begin - .\n" /* new instruction */ \
" .4byte " __stringify(ft_flags) "\n" /* feature + flags */ \
- " .byte " alt_total_slen "\n" /* source len */ \
- " .byte " alt_rlen "\n" /* replacement len */ \
+ " .byte " alt_total_slen(pfx) "\n" /* source len */ \
+ " .byte " alt_rlen(pfx) "\n" /* replacement len */ \
".popsection\n"
-#define ALTINSTR_REPLACEMENT(newinstr) /* replacement */ \
+#define ALTINSTR_REPLACEMENT(newinstr, pfx) /* replacement */ \
".pushsection .altinstr_replacement, \"ax\"\n" \
"# ALT: replacement\n" \
- "774:\n\t" newinstr "\n775:\n" \
+ #pfx "_alt_begin:\n\t" newinstr "\n" #pfx "_alt_end:\n" \
".popsection\n"
/* alternative assembly primitive: */
-#define ALTERNATIVE(oldinstr, newinstr, ft_flags) \
- OLDINSTR(oldinstr) \
- ALTINSTR_ENTRY(ft_flags) \
- ALTINSTR_REPLACEMENT(newinstr)
+#define __ALTERNATIVE(oldinstr, newinstr, ft_flags, pfx) \
+ OLDINSTR(oldinstr, pfx) \
+ ALTINSTR_ENTRY(ft_flags, pfx) \
+ ALTINSTR_REPLACEMENT(newinstr, pfx)
+
+#define ALTERNATIVE(oldinstr, newinstr, feat) \
+ __ALTERNATIVE(oldinstr, newinstr, feat, __UNIQUE_ID(altinstr))
#define ALTERNATIVE_2(oldinstr, newinstr1, ft_flags1, newinstr2, ft_flags2) \
ALTERNATIVE(ALTERNATIVE(oldinstr, newinstr1, ft_flags1), newinstr2, ft_flags2)
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 64ff73c533e5..d79552a61fda 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -163,7 +163,7 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
__asm__ ("" : "=r" (var) : "0" (var))
#endif
-#define __UNIQUE_ID(prefix) __PASTE(__PASTE(__UNIQUE_ID_, prefix), __COUNTER__)
+#define __UNIQUE_ID(prefix) __PASTE(__PASTE(__PASTE(__UNIQUE_ID_, prefix), _), __COUNTER__)
/**
* data_race - mark an expression as containing intentional data races
next prev parent reply other threads:[~2025-09-30 12:52 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-09-30 7:03 [PATCH v2 00/12] x86/msr: Inline rdmsr/wrmsr instructions Juergen Gross
2025-09-30 7:03 ` [PATCH v2 09/12] x86/msr: Use the alternatives mechanism for WRMSR Juergen Gross
2025-09-30 8:31 ` Peter Zijlstra
2025-09-30 8:46 ` Jürgen Groß
2025-09-30 8:50 ` Peter Zijlstra
2025-09-30 12:51 ` Peter Zijlstra [this message]
2025-09-30 15:42 ` Jürgen Groß
2025-10-01 6:43 ` Peter Zijlstra
2025-10-01 7:23 ` Peter Zijlstra
2025-10-03 14:23 ` Dave Hansen
2025-10-03 16:53 ` H. Peter Anvin
2025-10-01 8:49 ` Juergen Gross
2025-10-01 10:50 ` Peter Zijlstra
2025-10-01 11:16 ` Jürgen Groß
2025-09-30 16:00 ` Sean Christopherson
2025-10-01 9:13 ` Jürgen Groß
2025-09-30 19:19 ` [PATCH v2 00/12] x86/msr: Inline rdmsr/wrmsr instructions H. Peter Anvin
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