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Message-ID: <77f3841a92df5d0c819699ee3612118d566b7445.camel@intel.com>
Date: Thu, 27 Feb 2020 14:52:12 -0800
From: Yu-cheng Yu <yu-cheng.yu@...el.com>
To: Borislav Petkov <bp@...en8.de>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org,
"H. Peter Anvin" <hpa@...or.com>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Tony Luck <tony.luck@...el.com>,
Andy Lutomirski <luto@...nel.org>,
Rik van Riel <riel@...riel.com>,
"Ravi V. Shankar" <ravi.v.shankar@...el.com>,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
Fenghua Yu <fenghua.yu@...el.com>,
Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2 8/8] x86/fpu/xstate: Restore supervisor xstates for
__fpu__restore_sig()
On Fri, 2020-02-21 at 18:58 +0100, Borislav Petkov wrote:
> On Tue, Jan 21, 2020 at 12:18:43PM -0800, Yu-cheng Yu wrote:
> > [...]
> > arch/x86/kernel/fpu/signal.c | 26 +++++++++++++++++++++-----
> > 1 file changed, 21 insertions(+), 5 deletions(-)
> >
> > diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
> > index e3781a4a52a8..0d3e06a772b0 100644
> > --- a/arch/x86/kernel/fpu/signal.c
> > +++ b/arch/x86/kernel/fpu/signal.c
> > @@ -327,14 +327,22 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
> > }
> >
> > /*
> > - * The current state of the FPU registers does not matter. By setting
> > - * TIF_NEED_FPU_LOAD unconditionally it is ensured that the our xstate
> > - * is not modified on context switch and that the xstate is considered
> > + * Supervisor xstates are not modified by user space input, and
> > + * need to be saved and restored. Save the whole because doing
> > + * partial XSAVES changes xcomp_bv.
> > + * By setting TIF_NEED_FPU_LOAD it is ensured that our xstate is
> > + * not modified on context switch and that the xstate is considered
>
> Reflow those comments to 80 cols. There's room to the right.
>
> > * to be loaded again on return to userland (overriding last_cpu avoids
> > * the optimisation).
> > */
> > - set_thread_flag(TIF_NEED_FPU_LOAD);
> > + fpregs_lock();
> > + if (!test_thread_flag(TIF_NEED_FPU_LOAD)) {
> > + if (xfeatures_mask_supervisor())
> > + copy_xregs_to_kernel(&fpu->state.xsave);
> > + set_thread_flag(TIF_NEED_FPU_LOAD);
>
> So the code sets TIF_NEED_FPU_LOAD unconditionally, why are you changing
> this?
>
> Why don't you simply do:
>
> set_thread_flag(TIF_NEED_FPU_LOAD);
> fpregs_lock();
> if (xfeatures_mask_supervisor())
> copy_xregs_to_kernel(&fpu->state.xsave);
> fpregs_unlock();
If TIF_NEED_FPU_LOAD is set, then xstates are already in the xsave buffer.
We can skip saving them again.
Yu-cheng
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