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

From: Andrew Cooper
Date: Wed Feb 18 2015 - 05:57:37 EST

On 18/02/15 10:54, 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().

Ah yes. I was thinking in the wrong context for smp. In this case we
need to guarantee interdomain consistency even with a UP guest kernel.

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