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

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Date:   Thu, 13 Sep 2018 19:17:14 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Jason Andryuk <jandryuk@...il.com>
Cc:     stable@...r.kernel.org, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, jbeulich@...e.com,
        Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: Re: [PATCH 4.14] x86/pae: use 64 bit atomic xchg function in
 native_ptep_get_and_clear

On Thu, Sep 13, 2018 at 11:56:11AM -0400, Jason Andryuk wrote:
> 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>
> Atomic operations gained an arch_ prefix in commit
> 8bf705d130396e69c04cd8e6e010244ad2ce71f4
> s/arch_atomic64_xchg/atomic64_xchg/ for backport.
> Signed-off-by: Jason Andryuk <jandryuk@...il.com>

Thanks for the fix, I've now queued it up everywhere and will push out
-rc2 versions of this.

greg k-h