Re: [Xen-devel] [PATCH v5 1/4] xen: introduce a helper to allocate non-contiguous memory

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 11.05.15 at 11:52, <roger.pau@xxxxxxxxxx> wrote:
> El 11/05/15 a les 9.42, Jan Beulich ha escrit:
>>>>> On 08.05.15 at 16:34, <roger.pau@xxxxxxxxxx> wrote:
>>> +void vfree(void *va)
>>> +{
>>> +    unsigned int i, pages = vm_size(va);
>>> +
>>> +    if ( pages == 0 )
>>> +        return;
>>> +
>>> +    for ( i = 0; i < pages; i++ )
>>> +        free_domheap_pages(vmap_to_page(va + i * PAGE_SIZE), 1);
>>> +
>>> +    vunmap(va);
>>> +}
>> 
>> There actually is another problem here: You must not free the
>> pages before having torn down the mapping. An option (to
>> avoid the need for a memory allocation) might be to zap the
>> present bits (or whatever their equivalent on ARM) in a first
>> pass (making sure to flush TLBs), free, and then vunmap().
> 
> The aim of this is to catch reads/writes to those mappings while we are
> tearing them down?

No, that wouldn't be worth it. The aim is to avoid cache attribute
conflicts between the to be torn down mapping and possible
immediate re-uses of the pages elsewhere.

Jan


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