linux-kernel - Re: [PATCH] x86, fpu: do not BUG

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Message-ID: <20160720180256.GA14435@gmail.com>
Date:	Wed, 20 Jul 2016 20:02:56 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Dave Hansen <dave@...1.net>
Cc:	linux-kernel@...r.kernel.org, x86@...nel.org,
	dave.hansen@...ux.intel.com
Subject: Re: [PATCH] x86, fpu: do not BUG_ON() in early FPU code


* Dave Hansen <dave@...1.net> wrote:

> 
> From: Dave Hansen <dave.hansen@...ux.intel.com>
> 
> I don't think it is really possible to have a system where CPUID
> enumerates support for XSAVE but that it does not have FP/SSE
> (they are "legacy" features and always present).
> 
> But, I did manage to hit this case in qemu when I enabled its
> somewhat shaky XSAVE support.  The bummer is that the FPU is set
> up before we parse the command-line or have *any* console support
> including earlyprintk.
> 
> So a BUG() here is worthless.  All it does it guarantee that
> if/when we hit this case we have an empty console.  So, remove
> the BUG() and try to limp along by disabling XSAVE and trying to
> continue.
> 
> Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
> ---
> 
>  b/arch/x86/kernel/fpu/xstate.c |    3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff -puN arch/x86/kernel/fpu/xstate.c~x86-fpu-do-not-BUG_ON-early arch/x86/kernel/fpu/xstate.c
> --- a/arch/x86/kernel/fpu/xstate.c~x86-fpu-do-not-BUG_ON-early	2016-07-20 09:03:07.347941392 -0700
> +++ b/arch/x86/kernel/fpu/xstate.c	2016-07-20 09:17:15.001309137 -0700
> @@ -715,7 +715,8 @@ void __init fpu__init_system_xstate(void
>  
>  	if ((xfeatures_mask & XFEATURE_MASK_FPSSE) != XFEATURE_MASK_FPSSE) {
>  		pr_err("x86/fpu: FP/SSE not present amongst the CPU's xstate features: 0x%llx.\n", xfeatures_mask);
> -		BUG();
> +		fpu__init_disable_system_xstate();
> +		return;

Ok, but please put a comment there explaining why we are not stopping the system.

Thanks,

	Ingo