Re: [Xen-devel] [PATCHv4 04/14] xen: remove scratch frames for ballooned pages and m2p override

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 27/01/15 11:10, Stefano Stabellini wrote:
> On Tue, 27 Jan 2015, David Vrabel wrote:
>> On 27/01/15 10:57, Stefano Stabellini wrote:
>>> On Mon, 26 Jan 2015, David Vrabel wrote:
>>>> The scratch frame mappings for ballooned pages and the m2p override
>>>> are broken.  Remove them in preparation for replacing them with
>>>> simpler mechanisms that works.
>>>>
>>>> The scratch pages did not ensure that the page was not in use.  In
>>>> particular, the foreign page could still be in use by hardware.  If
>>>> the guest reused the frame the hardware could read or write that
>>>> frame.
>>>>
>>>> The m2p override did not handle the same frame being granted by two
>>>> different grant references.  Trying an M2P override lookup in this
>>>> case is impossible.
>>>>
>>>> Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
>>>
>>> [...]
>>>
>>>>  int set_foreign_p2m_mapping(struct gnttab_map_grant_ref *map_ops,
>>>>                        struct gnttab_map_grant_ref *kmap_ops,
>>>>                        struct page **pages, unsigned int count)
>>>>  {
>>>>    int i, ret = 0;
>>>> -  bool lazy = false;
>>>>    pte_t *pte;
>>>>  
>>>>    if (xen_feature(XENFEAT_auto_translated_physmap))
>>>>            return 0;
>>>>  
>>>> -  if (kmap_ops &&
>>>> -      !in_interrupt() &&
>>>> -      paravirt_get_lazy_mode() == PARAVIRT_LAZY_NONE) {
>>>> -          arch_enter_lazy_mmu_mode();
>>>> -          lazy = true;
>>>> +  if (kmap_ops) {
>>>> +          ret = HYPERVISOR_grant_table_op(GNTTABOP_map_grant_ref,
>>>> +                                          kmap_ops, count);
>>>> +          if (ret)
>>>> +                  goto out;
>>>>    }
>>>
>>> I think that you should explain why you are adding one more grant table
>>> mapping to a function meant to work on the p2m/m2p.  Everything else is
>>> clear.
>>
>> I'm not.  This function has always done this.
> 
> Can you be more specific please?
> 
> I cannot see a call to HYPERVISOR_grant_table_op among the lines you are
> removing from this function in this patch.

This function called m2p_add_override() which did the grant map
hypercall (an op at a time).

David
> 


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