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

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Date:   Fri, 16 Dec 2016 10:20:42 +0100
From:   Juergen Gross <jgross@...e.com>
To:     Borislav Petkov <bp@...e.de>
Cc:     Boris Ostrovsky <boris.ostrovsky@...cle.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 16/12/16 10:02, Borislav Petkov wrote:
> On Fri, Dec 16, 2016 at 08:28:46AM +0100, Juergen Gross wrote:
>> Not trying to load ucode in _any_ guest is an optimization only.
> 
> Does the hunk below work too?

Without testing, but I doubt it is working. As pv guests aren't coming
through check_loader_disabled_bsp() at all I can't see why your patch
would work for dom0. Additionally I don't think you want to call
native_cpuid() if have_cpuid_p() returns false.

So I think you want a generic "platform_allows_ucode_load()" function
checking for support of cpuid and virtualization. This function should
be called both in check_loader_disabled_bsp() and
check_loader_disabled_ap() to bail out early.


Juergen

> 
> I don't want to do hypervisor-specific solutions.
> 
> ---
> 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();
>  
>