linux-kernel - Re: [PATCHv14 08/17] x86/mm: Reduce untagged_addr() overhead until the first LAM user

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Date:   Tue, 17 Jan 2023 16:57:03 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>, x86@...nel.org,
        Kostya Serebryany <kcc@...gle.com>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Taras Madan <tarasmadan@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        "H . J . Lu" <hjl.tools@...il.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Bharata B Rao <bharata@....com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv14 08/17] x86/mm: Reduce untagged_addr() overhead until
 the first LAM user

On Tue, Jan 17, 2023 at 02:05:22PM +0100, Peter Zijlstra wrote:
> On Wed, Jan 11, 2023 at 03:37:27PM +0300, Kirill A. Shutemov wrote:
> 
> >  #define __untagged_addr(untag_mask, addr)
> >  	u64 __addr = (__force u64)(addr);				\
> > -	s64 sign = (s64)__addr >> 63;					\
> > -	__addr &= untag_mask | sign;					\
> > +	if (static_branch_likely(&tagged_addr_key)) {			\
> > +		s64 sign = (s64)__addr >> 63;				\
> > +		__addr &= untag_mask | sign;				\
> > +	}								\
> >  	(__force __typeof__(addr))__addr;				\
> >  })
> >  
> > #define untagged_addr(addr) __untagged_addr(current_untag_mask(), addr)
> 
> Is the compiler clever enough to put the memop inside the branch?

Hm. You mean current_untag_mask() inside static_branch_likely()?

But it is preprocessor who does this, not compiler. So, yes, the memop is
inside the branch.

Or I didn't understand your question.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov