linux-kernel - Re: [PATCH 11/22] x86/fpu: Make get_xsave_field_ptr() and get_xsave_addr() use feature number instead of mask

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Message-ID: <20190207111340.xaz7faetcags4qsb@linutronix.de>
Date:   Thu, 7 Feb 2019 12:13:40 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Borislav Petkov <bp@...en8.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 11/22] x86/fpu: Make get_xsave_field_ptr() and
 get_xsave_addr() use feature number instead of mask

On 2019-01-28 19:49:59 [+0100], Borislav Petkov wrote:
> > --- a/arch/x86/kernel/fpu/xstate.c
> > +++ b/arch/x86/kernel/fpu/xstate.c
> > @@ -830,15 +830,15 @@ static void *__raw_xsave_addr(struct xregs_state *xsave, int xfeature_nr)
…
> > -void *get_xsave_addr(struct xregs_state *xsave, int xstate_feature)
> > +void *get_xsave_addr(struct xregs_state *xsave, int xfeature_nr)
> >  {
> > -	int xfeature_nr;
> > +	u64 xfeature_mask = 1ULL << xfeature_nr;
> 
> You can paste directly BIT_ULL(xfeature_nr) where you need it in this
> function...
changed.

> > @@ -850,11 +850,11 @@ void *get_xsave_addr(struct xregs_state *xsave, int xstate_feature)
> >  	 * have not enabled.  Remember that pcntxt_mask is
> >  	 * what we write to the XCR0 register.
> >  	 */
> > -	WARN_ONCE(!(xfeatures_mask & xstate_feature),
> > +	WARN_ONCE(!(xfeatures_mask & xfeature_mask),
> 
> ... and turn this into:
> 
> 	WARN_ONCE(!(xfeatures_mask & BIT_ULL(xfeature_nr))
>
> which is more readable than the AND of two variables which I had to
> re-focus my eyes to see the difference. :)
> 
you mean with vs without the `s' ?

> Oh and this way, gcc generates better code by doing simply a BT
> directly:
> 
> # arch/x86/kernel/fpu/xstate.c:852:     WARN_ONCE(!(xfeatures_mask & BIT_ULL(xfeature_nr)),
>         .loc 1 852 2 view .LVU258
>         movq    xfeatures_mask(%rip), %rax      # xfeatures_mask, tmp124
>         btq     %rsi, %rax      # xfeature_nr, tmp124

interesting. gcc should know that it can use btq or shift + and because
it has all the raw data.
Anyway, I replaced the two user of xfeature_mask with
BIT_ULL(xfeature_nr).

> Thx.

Sebastian