linux-kernel - Re: Got FPU related warning on Intel Quark during boot

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Date:	Thu, 10 Mar 2016 15:59:40 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Andy Shevchenko <andy.shevchenko@...il.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Fenghua Yu <fenghua.yu@...el.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Oleg Nesterov <oleg@...hat.com>,
	"Yu, Yu-cheng" <yu-cheng.yu@...el.com>
Subject: Re: Got FPU related warning on Intel Quark during boot

On Thu, Mar 10, 2016 at 03:31:43PM +0200, Andy Shevchenko wrote:
> Looks like it lacks that one.
> 
> # grep -i fxsr /proc/cpuinfo; echo $?
> 1

Ok, so looking at where the warning comes from:

[   14.714533] WARNING: CPU: 0 PID: 823 at arch/x86/include/asm/fpu/internal.h:163 fpu__clear+0x8c/0x160

static inline void copy_kernel_to_fxregs(struct fxregs_state *fx)
{
        int err;

        if (config_enabled(CONFIG_X86_32)) {
                err = check_insn(fxrstor %[fx], "=m" (*fx), [fx] "m" (*fx));
		      ^^^^^^^^^^^^^^^^^
        } else {

	...

        /* Copying from a kernel buffer to FPU registers should never fail: */
        WARN_ON_FPU(err);


and the stacktrace is pretty clear:

flush_thread
|-> fpu__clear(&tsk->thread.fpu);
    |-> we are eager by default here:

        if (!use_eager_fpu() || !static_cpu_has(X86_FEATURE_FPU)) {
                /* FPU state will be reallocated lazily at the first use. */
                fpu__drop(fpu);
        } else {

		--> we're in that branch.

                copy_init_fpstate_to_fpregs();
		|-> copy_kernel_to_fxregs()


I think we should use FRSTOR on quark, i.e., copy_kernel_to_fregs().

Does this untested wild guess even work?

---
diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index dea8e76d60c6..bbafe5e8a1a6 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -474,8 +474,11 @@ static inline void copy_init_fpstate_to_fpregs(void)
 {
 	if (use_xsave())
 		copy_kernel_to_xregs(&init_fpstate.xsave, -1);
-	else
+	else if (static_cpu_has(X86_FEATURE_FXSR))
 		copy_kernel_to_fxregs(&init_fpstate.fxsave);
+	else
+		copy_kernel_to_fregs(&init_fpstate.fsave);
+
 }
 
 /*

-- 
Regards/Gruss,
    Boris.

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