linux-kernel - Re: [Xen-devel] [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 10:56:12 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Jan Beulich <JBeulich@...e.com>
Cc:     the arch/x86 maintainers <x86@...nel.org>, tglx@...utronix.de,
        xen-devel <xen-devel@...ts.xenproject.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>, mingo@...hat.com,
        linux-kernel@...r.kernel.org, hpa@...or.com
Subject: Re: [Xen-devel] [PATCH 2/2] x86/pae: use 64 bit atomic xchg function
 in native_ptep_get_and_clear

On 20/08/18 10:40, Jan Beulich wrote:
>>>> On 20.08.18 at 07:14, <jgross@...e.com> wrote:
>> @@ -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;
> 
> Why the union? pte_t already is one, with the pte field being what
> you're after ...
> 
>> -	/* 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);
> 
> ... here.

Uuh, yes.

I'm waiting for more comments, especially regarding the potential need
for a paravirt function.


Juergen