linux-kernel - Re: Can't boot as Xen dom0 due to commit fe055896

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Date:   Fri, 16 Dec 2016 10:01:00 +0100
From:   Borislav Petkov <bp@...e.de>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc:     Juergen Gross <jgross@...e.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        xen-devel <xen-devel@...ts.xenproject.org>
Subject: Re: Can't boot as Xen dom0 due to commit fe055896

On Thu, Dec 15, 2016 at 10:56:25PM -0500, Boris Ostrovsky wrote:
> You can use xen_cpuid_base(), for example. It will prevent microcode loading

Actually I want to do this at the end. CPUID(1).ECX[31] is reserved by
both vendors for hypervisor use.

> True. But I don't think it's clear that the problem we are seeing is
> Xen-specific.

If you can trigger it on baremetal, I'm all ears.

---
diff --git a/arch/x86/kernel/cpu/microcode/core.c b/arch/x86/kernel/cpu/microcode/core.c
index 6996413c78c3..54219f619205 100644
--- a/arch/x86/kernel/cpu/microcode/core.c
+++ b/arch/x86/kernel/cpu/microcode/core.c
@@ -76,6 +76,7 @@ struct cpu_info_ctx {
 static bool __init check_loader_disabled_bsp(void)
 {
 	static const char *__dis_opt_str = "dis_ucode_ldr";
+	u32 a, b, c, d;
 
 #ifdef CONFIG_X86_32
 	const char *cmdline = (const char *)__pa_nodebug(boot_command_line);
@@ -91,6 +92,17 @@ static bool __init check_loader_disabled_bsp(void)
 	if (cmdline_find_option_bool(cmdline, option))
 		*res = true;
 
+	if (!have_cpuid_p())
+		*res = true;
+
+	a = 1;
+	c = 0;
+	native_cpuid(&a, &b, &c, &d);
+
+	/* CPUID(1).ECX[31]: reserved for hypervisor use */
+	if (c & BIT(31))
+		*res = true;
+
 	return *res;
 }
 
@@ -121,9 +133,6 @@ void __init load_ucode_bsp(void)
 	if (check_loader_disabled_bsp())
 		return;
 
-	if (!have_cpuid_p())
-		return;
-
 	vendor = x86_cpuid_vendor();
 	family = x86_cpuid_family();
 
@@ -157,9 +166,6 @@ void load_ucode_ap(void)
 	if (check_loader_disabled_ap())
 		return;
 
-	if (!have_cpuid_p())
-		return;
-
 	vendor = x86_cpuid_vendor();
 	family = x86_cpuid_family();
 

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--