linux-kernel - Re: [PATCH 2/2] x86/pae: use 64 bit atomic xchg function in native_ptep_get_and

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Date:   Mon, 20 Aug 2018 15:26:07 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Juergen Gross <jgross@...e.com>
cc:     linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org,
        x86@...nel.org, boris.ostrovsky@...cle.com, hpa@...or.com,
        mingo@...hat.com
Subject: Re: [PATCH 2/2] x86/pae: use 64 bit atomic xchg function in
 native_ptep_get_and_clear

On Mon, 20 Aug 2018, Juergen Gross wrote:
> In case adding about 6 cycles for native_ptep_get_and_clear() is believed
> to be too bad I can modify the patch to add a paravirt function for that
> purpose in order to add the overhead for Xen guests only (in fact the
> overhead for Xen guests will be less, as only one instruction writing to
> the PTE has to be emulated by the hypervisor).

I doubt that its worth the trouble of yet another paravirt thingy.

> ---
>  arch/x86/include/asm/pgtable-3level.h | 14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/include/asm/pgtable-3level.h b/arch/x86/include/asm/pgtable-3level.h
> index a564084c6141..7919ae4e27d8 100644
> --- a/arch/x86/include/asm/pgtable-3level.h
> +++ b/arch/x86/include/asm/pgtable-3level.h
> @@ -2,6 +2,8 @@
>  #ifndef _ASM_X86_PGTABLE_3LEVEL_H
>  #define _ASM_X86_PGTABLE_3LEVEL_H
>  
> +#include <asm/atomic64_32.h>
> +
>  /*
>   * Intel Physical Address Extension (PAE) Mode - three-level page
>   * tables on PPro+ CPUs.
> @@ -148,14 +150,14 @@ static inline void pud_clear(pud_t *pudp)
>  #ifdef CONFIG_SMP
>  static inline pte_t native_ptep_get_and_clear(pte_t *ptep)
>  {
> -	pte_t res;
> +	union {
> +		pte_t pte;
> +		long long val;
> +	} res;
>  
> -	/* xchg acts as a barrier before the setting of the high bits */
> -	res.pte_low = xchg(&ptep->pte_low, 0);
> -	res.pte_high = ptep->pte_high;
> -	ptep->pte_high = 0;
> +	res.val = arch_atomic64_xchg((atomic64_t *)ptep, 0);

Couldn't you just keep

	 pte_t res;

and do:

	res.pte = (pteval_t) arch_atomic64_xchg((atomic64_t *)ptep, 0);

Hmm?

Thanks,

	tglx