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Date: Wed, 18 Feb 2015 10:54:08 +0000
From: David Vrabel <david.vrabel@citrix.com>
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To: Andrew Cooper <andrew.cooper3@citrix.com>, Juergen Gross <jgross@suse.com>,
        <linux-kernel@vger.kernel.org>, <xen-devel@lists.xensource.com>,
        <konrad.wilk@oracle.com>, <boris.ostrovsky@oracle.com>
Subject: Re: [Xen-devel] [PATCH 02/13] xen: anchor linear p2m list in shared
 info structure
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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().

David