linux-kernel - Re: [tip: x86/asm] x86/kexec: Merge x86_32 and x86

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Message-ID: <36B61764-A297-459A-AD55-ACC54C409876@zytor.com>
Date: Thu, 06 Mar 2025 19:00:35 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: linux-kernel@...r.kernel.org,
        tip-bot2 for Uros Bizjak <tip-bot2@...utronix.de>,
        linux-tip-commits@...r.kernel.org
CC: Uros Bizjak <ubizjak@...il.com>, Ingo Molnar <mingo@...nel.org>,
        David Woodhouse <dwmw@...zon.co.uk>, Baoquan He <bhe@...hat.com>,
        Vivek Goyal <vgoyal@...hat.com>, Dave Young <dyoung@...hat.com>,
        Ard Biesheuvel <ardb@...nel.org>, x86@...nel.org
Subject: Re: [tip: x86/asm] x86/kexec: Merge x86_32 and x86_64 code using macros from <asm/asm.h>

On March 6, 2025 1:33:43 PM PST, tip-bot2 for Uros Bizjak <tip-bot2@...utronix.de> wrote:
>The following commit has been merged into the x86/asm branch of tip:
>
>Commit-ID:     aa3942d4d12ef57f031faa2772fe410c24191e36
>Gitweb:        https://git.kernel.org/tip/aa3942d4d12ef57f031faa2772fe410c24191e36
>Author:        Uros Bizjak <ubizjak@...il.com>
>AuthorDate:    Thu, 06 Mar 2025 15:52:11 +01:00
>Committer:     Ingo Molnar <mingo@...nel.org>
>CommitterDate: Thu, 06 Mar 2025 22:04:48 +01:00
>
>x86/kexec: Merge x86_32 and x86_64 code using macros from <asm/asm.h>
>
>Merge common x86_32 and x86_64 code in crash_setup_regs()
>using macros from <asm/asm.h>.
>
>The compiled object files before and after the patch are unchanged.
>
>Signed-off-by: Uros Bizjak <ubizjak@...il.com>
>Signed-off-by: Ingo Molnar <mingo@...nel.org>
>Cc: David Woodhouse <dwmw@...zon.co.uk>
>Cc: Baoquan He <bhe@...hat.com>
>Cc: Vivek Goyal <vgoyal@...hat.com>
>Cc: Dave Young <dyoung@...hat.com>
>Cc: Ard Biesheuvel <ardb@...nel.org>
>Cc: "H. Peter Anvin" <hpa@...or.com>
>Link: https://lore.kernel.org/r/20250306145227.55819-1-ubizjak@gmail.com
>---
> arch/x86/include/asm/kexec.h | 58 +++++++++++++++--------------------
> 1 file changed, 25 insertions(+), 33 deletions(-)
>
>diff --git a/arch/x86/include/asm/kexec.h b/arch/x86/include/asm/kexec.h
>index 8ad1874..e3589d6 100644
>--- a/arch/x86/include/asm/kexec.h
>+++ b/arch/x86/include/asm/kexec.h
>@@ -18,6 +18,7 @@
> #include <linux/string.h>
> #include <linux/kernel.h>
> 
>+#include <asm/asm.h>
> #include <asm/page.h>
> #include <asm/ptrace.h>
> 
>@@ -71,41 +72,32 @@ static inline void crash_setup_regs(struct pt_regs *newregs,
> 	if (oldregs) {
> 		memcpy(newregs, oldregs, sizeof(*newregs));
> 	} else {
>+		asm volatile("mov %%" _ASM_BX ",%0" : "=m"(newregs->bx));
>+		asm volatile("mov %%" _ASM_CX ",%0" : "=m"(newregs->cx));
>+		asm volatile("mov %%" _ASM_DX ",%0" : "=m"(newregs->dx));
>+		asm volatile("mov %%" _ASM_SI ",%0" : "=m"(newregs->si));
>+		asm volatile("mov %%" _ASM_DI ",%0" : "=m"(newregs->di));
>+		asm volatile("mov %%" _ASM_BP ",%0" : "=m"(newregs->bp));
>+		asm volatile("mov %%" _ASM_AX ",%0" : "=m"(newregs->ax));
>+		asm volatile("mov %%" _ASM_SP ",%0" : "=m"(newregs->sp));
>+#ifdef CONFIG_X86_64
>+		asm volatile("mov %%r8,%0" : "=m"(newregs->r8));
>+		asm volatile("mov %%r9,%0" : "=m"(newregs->r9));
>+		asm volatile("mov %%r10,%0" : "=m"(newregs->r10));
>+		asm volatile("mov %%r11,%0" : "=m"(newregs->r11));
>+		asm volatile("mov %%r12,%0" : "=m"(newregs->r12));
>+		asm volatile("mov %%r13,%0" : "=m"(newregs->r13));
>+		asm volatile("mov %%r14,%0" : "=m"(newregs->r14));
>+		asm volatile("mov %%r15,%0" : "=m"(newregs->r15));
>+#endif
>+		asm volatile("mov %%ss,%k0" : "=a"(newregs->ss));
>+		asm volatile("mov %%cs,%k0" : "=a"(newregs->cs));
> #ifdef CONFIG_X86_32
>-		asm volatile("movl %%ebx,%0" : "=m"(newregs->bx));
>-		asm volatile("movl %%ecx,%0" : "=m"(newregs->cx));
>-		asm volatile("movl %%edx,%0" : "=m"(newregs->dx));
>-		asm volatile("movl %%esi,%0" : "=m"(newregs->si));
>-		asm volatile("movl %%edi,%0" : "=m"(newregs->di));
>-		asm volatile("movl %%ebp,%0" : "=m"(newregs->bp));
>-		asm volatile("movl %%eax,%0" : "=m"(newregs->ax));
>-		asm volatile("movl %%esp,%0" : "=m"(newregs->sp));
>-		asm volatile("movl %%ss, %%eax;" :"=a"(newregs->ss));
>-		asm volatile("movl %%cs, %%eax;" :"=a"(newregs->cs));
>-		asm volatile("movl %%ds, %%eax;" :"=a"(newregs->ds));
>-		asm volatile("movl %%es, %%eax;" :"=a"(newregs->es));
>-		asm volatile("pushfl; popl %0" :"=m"(newregs->flags));
>-#else
>-		asm volatile("movq %%rbx,%0" : "=m"(newregs->bx));
>-		asm volatile("movq %%rcx,%0" : "=m"(newregs->cx));
>-		asm volatile("movq %%rdx,%0" : "=m"(newregs->dx));
>-		asm volatile("movq %%rsi,%0" : "=m"(newregs->si));
>-		asm volatile("movq %%rdi,%0" : "=m"(newregs->di));
>-		asm volatile("movq %%rbp,%0" : "=m"(newregs->bp));
>-		asm volatile("movq %%rax,%0" : "=m"(newregs->ax));
>-		asm volatile("movq %%rsp,%0" : "=m"(newregs->sp));
>-		asm volatile("movq %%r8,%0" : "=m"(newregs->r8));
>-		asm volatile("movq %%r9,%0" : "=m"(newregs->r9));
>-		asm volatile("movq %%r10,%0" : "=m"(newregs->r10));
>-		asm volatile("movq %%r11,%0" : "=m"(newregs->r11));
>-		asm volatile("movq %%r12,%0" : "=m"(newregs->r12));
>-		asm volatile("movq %%r13,%0" : "=m"(newregs->r13));
>-		asm volatile("movq %%r14,%0" : "=m"(newregs->r14));
>-		asm volatile("movq %%r15,%0" : "=m"(newregs->r15));
>-		asm volatile("movl %%ss, %%eax;" :"=a"(newregs->ss));
>-		asm volatile("movl %%cs, %%eax;" :"=a"(newregs->cs));
>-		asm volatile("pushfq; popq %0" :"=m"(newregs->flags));
>+		asm volatile("mov %%ds,%k0" : "=a"(newregs->ds));
>+		asm volatile("mov %%es,%k0" : "=a"(newregs->es));
> #endif
>+		asm volatile("pushf\n\t"
>+			     "pop %0" : "=m"(newregs->flags));
> 		newregs->ip = _THIS_IP_;
> 	}
> }

Incidentally, doing this in C code is obviously completely broken, especially doing it in multiple statements. You have no idea what the compiler has messed with before you get there.