linux-kernel - Re: [PATCH 10/11] x86/fpu: prepare copy_fpstate_to_sigframe for TIF_LOAD

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Message-ID: <20181102225508.nv552gj356aykahf@linutronix.de>
Date:   Fri, 2 Nov 2018 23:55:08 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
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>
Subject: Re: [PATCH 10/11] x86/fpu: prepare copy_fpstate_to_sigframe for
 TIF_LOAD_FPU

On 2018-10-12 12:40:19 [-0700], Dave Hansen wrote:
> On 10/04/2018 07:05 AM, Sebastian Andrzej Siewior wrote:
> > From: Rik van Riel <riel@...riel.com>
> > 
> > If TIF_LOAD_FPU is set, then the registers are saved (not loaded). In that case
> > we skip the saving part.
> 
> This sentence hurts my brain.
> 
> 	"If TIF_LOAD_FPU is set the registers are ... not loaded"
> 
> I think that means that something could use some better naming.
> 
> Should TIF_LOAD_FPU be TIF_NEED_FPU_LOAD, perhaps?

ARM has TIF_FOREIGN_FPSTATE which basically means that the current FP
stat does not belong to current().
Let me try to use TIF_NEED_FPU_LOAD and see how that works out.

> > Signed-off-by: Rik van Riel <riel@...riel.com>
> > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> > ---
> >  arch/x86/kernel/fpu/signal.c | 16 ++++++++++------
> >  1 file changed, 10 insertions(+), 6 deletions(-)
> > 
> > diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
> > index c8f5ff58578ed..979dcd1ed82e0 100644
> > --- a/arch/x86/kernel/fpu/signal.c
> > +++ b/arch/x86/kernel/fpu/signal.c
> > @@ -155,13 +155,17 @@ int copy_fpstate_to_sigframe(void __user *buf, void __user *buf_fx, int size)
> >  			sizeof(struct user_i387_ia32_struct), NULL,
> >  			(struct _fpstate_32 __user *) buf) ? -1 : 1;
> >  
> > -	/* Update the thread's fxstate to save the fsave header. */
> > -	if (ia32_fxstate) {
> > -		copy_fxregs_to_kernel(fpu);
> > -	} else {
> > -		copy_fpregs_to_fpstate(fpu);
> > -		fpregs_deactivate(fpu);
> > +	__fpregs_changes_begin();
> > +	if (!test_thread_flag(TIF_LOAD_FPU)) {
> 
> This needs commenting, please.
> 
> If we do not need to load the FPU at return to userspace, it means the
> state is in the the registers, not the buffer.  So, update the buffer to
> match the registers.

okay. Let me add this then.

Sebastian