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 16:55:57 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Thomas Gleixner <tglx@...utronix.de>
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 20/08/18 15:26, Thomas Gleixner wrote:
> 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?

Yes, got this suggestion already by Jan. I'm waiting with V2 until
tomorrow to see whether someone has other complaints.


Juergen