linux-kernel - Re: [Xen-devel] [PATCH 02/13] xen: anchor linear p2m list in shared info structure

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Date:	Wed, 18 Feb 2015 11:57:45 +0100
From:	Juergen Gross <jgross@...e.com>
To:	David Vrabel <david.vrabel@...rix.com>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	linux-kernel@...r.kernel.org, xen-devel@...ts.xensource.com,
	konrad.wilk@...cle.com, boris.ostrovsky@...cle.com
Subject: Re: [Xen-devel] [PATCH 02/13] xen: anchor linear p2m list in shared
 info structure

On 02/18/2015 11:54 AM, David Vrabel wrote:
> On 18/02/15 10:50, Andrew Cooper wrote:
>> On 18/02/15 10:42, Juergen Gross wrote:
>>>
>>>>>    /* Set up p2m_top to point to the domain-builder provided p2m
>>>>> pages */
>>>>> @@ -469,8 +473,10 @@ static pte_t *alloc_p2m_pmd(unsigned long addr,
>>>>> pte_t *pte_pg)
>>>>>
>>>>>            ptechk = lookup_address(vaddr, &level);
>>>>>            if (ptechk == pte_pg) {
>>>>> +            HYPERVISOR_shared_info->arch.p2m_generation++;
>>>>>                set_pmd(pmdp,
>>>>>                    __pmd(__pa(pte_newpg[i]) | _KERNPG_TABLE));
>>>>> +            HYPERVISOR_shared_info->arch.p2m_generation++;
>>>>
>>>> Do these increments of p2m_generation need to be atomic?
>>>
>>> Hmm, they are done under lock. I don't think the compiler is allowed to
>>> reorder the writes to p2m_generation across set_pmd().
>>
>> They do need smp_wmb() to guarantee that the increment is visible before
>> the update occurs, just as the toolstack will need smp_rmb() to read.
>
> smp_wmb() isn't good enough since you need the barrier even on non-smp
> -- you need a wmb().

Okay, will do.

Juergen

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