Date:	Thu, 20 Feb 2014 18:28:09 +0000
From:	David Vrabel <david.vrabel@...rix.com>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
CC:	Zoltan Kiss <zoltan.kiss@...rix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, <x86@...nel.org>,
	<xen-devel@...ts.xenproject.org>, <linux-kernel@...r.kernel.org>,
	Roger Pau Monné <roger.pau@...rix.com>,
	Jan Beulich <jbeulich@...e.com>,
	Ian Campbell <Ian.Campbell@...rix.com>
Subject: Re: [PATCH v9] xen/grant-table: Avoid m2p_override during mapping

On 20/02/14 18:26, Stefano Stabellini wrote:
> On Thu, 20 Feb 2014, David Vrabel wrote:
>> On 20/02/14 18:17, Stefano Stabellini wrote:
>>> On Thu, 20 Feb 2014, Zoltan Kiss wrote:
>>>> On 20/02/14 17:26, Stefano Stabellini wrote:
>>>>> On Mon, 17 Feb 2014, Zoltan Kiss wrote:
>>>>>> On 16/02/14 18:36, Stefano Stabellini wrote:
>>>>>>> On Wed, 12 Feb 2014, Zoltan Kiss wrote:
>>>>>>>> diff --git a/arch/arm/include/asm/xen/page.h
>>>>>>>> b/arch/arm/include/asm/xen/page.h
>>>>>>>> index e0965ab..4eaeb3f 100644
>>>>>>>> --- a/arch/arm/include/asm/xen/page.h
>>>>>>>> +++ b/arch/arm/include/asm/xen/page.h
>>>>>>>> @@ -97,16 +97,15 @@ static inline pte_t *lookup_address(unsigned long
>>>>>>>> address, unsigned int *level)
>>>>>>>>    	return NULL;
>>>>>>>>    }
>>>>>>>>
>>>>>>>> -static inline int m2p_add_override(unsigned long mfn, struct page
>>>>>>>> *page,
>>>>>>>> -		struct gnttab_map_grant_ref *kmap_op)
>>>>>>>> -{
>>>>>>>> -	return 0;
>>>>>>>> -}
>>>>>>>> -
>>>>>>>> -static inline int m2p_remove_override(struct page *page, bool
>>>>>>>> clear_pte)
>>>>>>>> -{
>>>>>>>> -	return 0;
>>>>>>>> -}
>>>>>>>> +extern 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,
>>>>>>>> +				   bool m2p_override);
>>>>>>>> +
>>>>>>>> +extern int clear_foreign_p2m_mapping(struct gnttab_unmap_grant_ref
>>>>>>>> *unmap_ops,
>>>>>>>> +				     struct gnttab_map_grant_ref
>>>>>>>> *kmap_ops,
>>>>>>>> +				     struct page **pages, unsigned int
>>>>>>>> count,
>>>>>>>> +				     bool m2p_override);
>>>>>>>
>>>>>>> Much much better.
>>>>>>> The only comment I have is about this m2p_override boolean parameter.
>>>>>>> m2p_override is now meaningless in this context, what we really want to
>>>>>>> let the arch specific implementation know is whether the mapping is a
>>>>>>> kernel only mapping or a userspace mapping.
>>>>>>> Testing for kmap_ops != NULL might even be enough, but it would not
>>>>>>> improve the interface.
>>>>>> gntdev is the only user of this, the kmap_ops parameter there is:
>>>>>> use_ptemod ? map->kmap_ops + offset : NULL
>>>>>> where:
>>>>>> use_ptemod = !xen_feature(XENFEAT_auto_translated_physmap);
>>>>>> So I think we can't rely on kmap_ops to decide whether we should
>>>>>> m2p_override
>>>>>> or not.
>>>>>>
>>>>>>> Is it possible to realize if the mapping is a userspace mapping by
>>>>>>> checking for GNTMAP_application_map in map_ops?
>>>>>>> Otherwise I would keep the boolean and rename it to user_mapping.
>>>>>> Sounds better, but as far as I see gntdev set that flag in
>>>>>> find_grant_ptes,
>>>>>> which is called only
>>>>>>
>>>>>> if (use_ptemod) {
>>>>>> 	err = apply_to_page_range(vma->vm_mm, vma->vm_start,
>>>>>> 				  vma->vm_end - vma->vm_start,
>>>>>> 				  find_grant_ptes, map);
>>>>>>
>>>>>> So if xen_feature(XENFEAT_auto_translated_physmap), we don't have
>>>>>> kmap_ops,
>>>>>> and GNTMAP_application_map is not set as well, but I guess we still need
>>>>>> m2p_override. Or not? I'm a bit confused, maybe because of Monday ...
>>>>>
>>>>> If xen_feature(XENFEAT_auto_translated_physmap) we shouldn't need the
>>>>> m2p_override.
>>>>>
>>>>
>>>> So it's safe to assume that we need m2p_override only if kmap_ops != NULL, and
>>>> we can avoid the extra bool parameter, is that correct? At least with the
>>>> current users of grant mapping it seems to be true.
>>>> In which case we don't need the wrappers for gnttab_[un]map_refs as well.
>>>> Actually the most of m2p_add/remove_override takes effect only if there is a
>>>> kmap_op parameter, but what about the rest of the code there?
>>>
>>> It is safe to assume that we only need the m2p_override if
>>> !xen_feature(XENFEAT_auto_translated_physmap).
>>> I wouldn't make any assumptions on kmap_ops != NULL.
>>
>> I think it is -- we only need the m2p override if we have userspace
>> mappings (where kmap_ops != 0).
>>
>>> I would remove the bool m2p_override parameter completely and determine
>>> whether we need to call the m2p_override functions from the x86
>>> implementation of set/clear_foreign_p2m_mapping by checking
>>> xen_feature(XENFEAT_auto_translated_physmap).
>>>
>>> David, does it seem reasonable to you?
>>
>> That would miss the point of this patch which is to avoid adding to the
>> m2p_override for kernel only mappings.
> 
> I meant checking 
> 
> !xen_feature(XENFEAT_auto_translated_physmap) && kmap_ops != 0
> 
> At least this way the "hack" would be entirely self contained in the
> arch specific code.

Ok. That would work.

David

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