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

["Roger Pau Monné" <roger.pau@citrix.com>]

Hello,

El 11/05/15 a les 9.42, Jan Beulich ha escrit:
>>>> On 08.05.15 at 16:34, <roger.pau@citrix.com> 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?

I could do that on x86 without problems using the present bit in the
pte, but I'm not sure how to do it on ARM because AFAICT there's no
present bit. The valid bit on ARM looks like an option, but then I don't
know if va_to_par is going to fail because the valid bit is not set.

Roger.