linux-kernel - Re: [PATCH 4.14 102/115] x86/pae: use 64 bit atomic xchg function in native_ptep_get_and

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Date:   Thu, 13 Sep 2018 10:55:08 -0400
From:   Jason Andryuk <jandryuk@...il.com>
To:     gregkh@...uxfoundation.org
Cc:     open list <linux-kernel@...r.kernel.org>, stable@...r.kernel.org,
        Juergen Gross <jgross@...e.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jan Beulich <jbeulich@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: Re: [PATCH 4.14 102/115] x86/pae: use 64 bit atomic xchg function in native_ptep_get_and_clear

On Thu, Sep 13, 2018 at 9:48 AM Greg Kroah-Hartman
<gregkh@...uxfoundation.org> wrote:
>
> 4.14-stable review patch.  If anyone has any objections, please let me know.
>
> ------------------
>
> From: Juergen Gross <jgross@...e.com>
>
> commit b2d7a075a1ccef2fb321d595802190c8e9b39004 upstream.
>
> Using only 32-bit writes for the pte will result in an intermediate
> L1TF vulnerable PTE. When running as a Xen PV guest this will at once
> switch the guest to shadow mode resulting in a loss of performance.
>
> Use arch_atomic64_xchg() instead which will perform the requested
> operation atomically with all 64 bits.
>
> Some performance considerations according to:
>
> https://software.intel.com/sites/default/files/managed/ad/dc/Intel-Xeon-Scalable-Processor-throughput-latency.pdf
>
> The main number should be the latency, as there is no tight loop around
> native_ptep_get_and_clear().
>
> "lock cmpxchg8b" has a latency of 20 cycles, while "lock xchg" (with a
> memory operand) isn't mentioned in that document. "lock xadd" (with xadd
> having 3 cycles less latency than xchg) has a latency of 11, so we can
> assume a latency of 14 for "lock xchg".
>
> Signed-off-by: Juergen Gross <jgross@...e.com>
> Reviewed-by: Thomas Gleixner <tglx@...utronix.de>
> Reviewed-by: Jan Beulich <jbeulich@...e.com>
> Tested-by: Jason Andryuk <jandryuk@...il.com>
> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>
> Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
>
> ---
>  arch/x86/include/asm/pgtable-3level.h |    7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
>
> --- 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.
> @@ -147,10 +149,7 @@ static inline pte_t native_ptep_get_and_
>  {
>         pte_t 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.pte = (pteval_t)arch_atomic64_xchg((atomic64_t *)ptep, 0);

For 4.14, I had to change this to atomic64_xchg since
arch_atomic64_xchg doesn't exist.

kernel-source/arch/x86/include/asm/pgtable-3level.h:152:22: error:
implicit declaration of function 'arch_atomic64_xchg'
[-Werror=implicit-function-declaration]

The same is probably needed for earlier versions as well.

Regards,
Jason

>
>         return res;
>  }
>
>