From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Jan Beulich" <JBeulich@suse.com>
Subject: Re: [PATCH v5 1/4] xen: introduce a helper to allocate
 non-contiguous memory
Date: Mon, 11 May 2015 11:41:29 +0100
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To: =?UTF-8?Q?Roger=20Pau=20Monn=C3=A9?= <roger.pau@citrix.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>, Tim Deegan <tim@xen.org>, Stefano Stabellini <stefano.stabellini@citrix.com>, Ian Campbell <ian.campbell@citrix.com>, xen-devel@lists.xenproject.org
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>>> On 11.05.15 at 11:52, <roger.pau@citrix.com> wrote:
> 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?

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