linux-kernel - Re: [PATCH] x86/microcode: Fix crashes on early 486 CPUs due to usage of 'cpuid'.

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Message-ID: <20250405093127.GAZ_D4b6NdyTS-UW1J@fat_crate.local>
Date: Sat, 5 Apr 2025 11:32:26 +0200
From: Borislav Petkov <bp@...en8.de>
To: Kevin Koster <lkml@...ertech.com>, Oerg866 <oerg866@...glemail.com>
Cc: linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] x86/microcode: Fix crashes on early 486 CPUs due to
 usage of 'cpuid'.

On Sat, Apr 05, 2025 at 01:03:06PM +1100, Kevin Koster wrote:
> On Sat, 19 Oct 2024 08:29:04 +0200
> Oerg866 <oerg866@...glemail.com> wrote:
> 
> > Starting with v6.7-rc1, the kernel was no longer able to boot on early
> > i486-class CPUs.
> 
> Thanks for this patch! It solves my problem with kernel 6.12.11
> rebooting at start-up on 486 CPUs, which had me puzzled. (tested on
> AM486DX2-66 and CX486DX4-100)
> 
> Is there a reason why the patch wasn't accepted?

Yes, too many patches, too little time. :-(

Anyway, does the one below - only build-tested - work for both y'all too?

---
diff --git a/arch/x86/include/asm/microcode.h b/arch/x86/include/asm/microcode.h
index 695e569159c1..d53148fb893a 100644
--- a/arch/x86/include/asm/microcode.h
+++ b/arch/x86/include/asm/microcode.h
@@ -17,10 +17,12 @@ struct ucode_cpu_info {
 void load_ucode_bsp(void);
 void load_ucode_ap(void);
 void microcode_bsp_resume(void);
+bool __init microcode_loader_disabled(void);
 #else
 static inline void load_ucode_bsp(void)	{ }
 static inline void load_ucode_ap(void) { }
 static inline void microcode_bsp_resume(void) { }
+bool __init microcode_loader_disabled(void) { return false; }
 #endif
 
 extern unsigned long initrd_start_early;
diff --git a/arch/x86/kernel/cpu/microcode/amd.c b/arch/x86/kernel/cpu/microcode/amd.c
index b61028cf5c8a..dda7f0d409e9 100644
--- a/arch/x86/kernel/cpu/microcode/amd.c
+++ b/arch/x86/kernel/cpu/microcode/amd.c
@@ -1099,7 +1099,7 @@ static int __init save_microcode_in_initrd(void)
 	enum ucode_state ret;
 	struct cpio_data cp;
 
-	if (dis_ucode_ldr || c->x86_vendor != X86_VENDOR_AMD || c->x86 < 0x10)
+	if (microcode_loader_disabled() || c->x86_vendor != X86_VENDOR_AMD || c->x86 < 0x10)
 		return 0;
 
 	if (!find_blobs_in_containers(&cp))
diff --git a/arch/x86/kernel/cpu/microcode/core.c b/arch/x86/kernel/cpu/microcode/core.c
index b3658d11e7b6..972338a2abae 100644
--- a/arch/x86/kernel/cpu/microcode/core.c
+++ b/arch/x86/kernel/cpu/microcode/core.c
@@ -95,12 +95,15 @@ static bool amd_check_current_patch_level(void)
 	return false;
 }
 
-static bool __init check_loader_disabled_bsp(void)
+bool __init microcode_loader_disabled(void)
 {
 	static const char *__dis_opt_str = "dis_ucode_ldr";
 	const char *cmdline = boot_command_line;
 	const char *option  = __dis_opt_str;
 
+	if (!have_cpuid_p())
+		return true;
+
 	/*
 	 * CPUID(1).ECX[31]: reserved for hypervisor use. This is still not
 	 * completely accurate as xen pv guests don't see that CPUID bit set but
@@ -146,7 +149,7 @@ void __init load_ucode_bsp(void)
 		return;
 	}
 
-	if (check_loader_disabled_bsp())
+	if (microcode_loader_disabled())
 		return;
 
 	if (intel)
diff --git a/arch/x86/kernel/head32.c b/arch/x86/kernel/head32.c
index de001b2146ab..f29dc9c95c50 100644
--- a/arch/x86/kernel/head32.c
+++ b/arch/x86/kernel/head32.c
@@ -145,8 +145,7 @@ void __init __no_stack_protector mk_early_pgtbl_32(void)
 	*ptr = (unsigned long)ptep + PAGE_OFFSET;
 
 #ifdef CONFIG_MICROCODE_INITRD32
-	/* Running on a hypervisor? */
-	if (native_cpuid_ecx(1) & BIT(31))
+	if (microcode_loader_disabled())
 		return;
 
 	params = (struct boot_params *)__pa_nodebug(&boot_params);

-- 
Regards/Gruss,
    Boris.

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