linux-kernel - Re: [PATCH 18/24] x86/fpu: Prepare copy_fpstate_to_sigframe() for TIF_NEED_FPU

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Date:   Mon, 1 Apr 2019 10:24:24 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        kvm@...r.kernel.org, "Jason A. Donenfeld" <Jason@...c4.com>,
        Rik van Riel <riel@...riel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH 18/24] x86/fpu: Prepare copy_fpstate_to_sigframe() for
 TIF_NEED_FPU_LOAD

On 2019-03-31 20:20:25 [+0200], Thomas Gleixner wrote:
> 
> I think this should do the following:
> 
> 	fpregs_lock();
> 	if (!test_thread_flag(TIF_NEED_FPU_LOAD)) {
> 		pagefault_disable();
> 		ret = copy_fpu_to_user(...);
> 		pagefault_enable();
> 		if (!res)
> 			return 0;
> 		copy_fpregs_to_fpstate(fpu); 
> 	}
> 	fpregs_unlock();
> 
> The point is that in most cases the direct store from the FPU registers to
> user space will succeed simply because the stack is accessible and you only
> do the store in kernel memory and copy when that fails.

let me try that.

> Thanks,
> 
> 	tglx
> 

Sebastian