Re: [PATCH V4] xen/gnttab: Store frame GFN in struct page_info on Arm

[Jan Beulich <jbeulich@xxxxxxxx>]

On 22.12.2021 13:44, Oleksandr wrote:
> 
> On 22.12.21 14:33, Julien Grall wrote:
>> Hi Jan,
> 
> 
> Hi Julien, Jan
> 
> 
> 
>>
>> On 22/12/2021 13:05, Jan Beulich wrote:
>>> On 22.12.2021 11:01, Julien Grall wrote:
>>>> On 14/12/2021 17:45, Jan Beulich wrote:
>>>>> On 14.12.2021 17:26, Oleksandr wrote:
>>>>>> On 14.12.21 15:37, Jan Beulich wrote:
>>>>>>> On 03.12.2021 21:33, Oleksandr Tyshchenko wrote:
>>>>>>>> @@ -2177,14 +2181,22 @@ void *alloc_xenheap_pages(unsigned int 
>>>>>>>> order, unsigned int memflags)
>>>>>>>>         void free_xenheap_pages(void *v, unsigned int order)
>>>>>>>>     {
>>>>>>>> +    struct page_info *pg;
>>>>>>>> +    unsigned int i;
>>>>>>>> +
>>>>>>>>         ASSERT(!in_irq());
>>>>>>>>             if ( v == NULL )
>>>>>>>>             return;
>>>>>>>>     +    pg = virt_to_page(v);
>>>>>>>> +
>>>>>>>>         memguard_guard_range(v, 1 << (order + PAGE_SHIFT));
>>>>>>> ... this really want to (logically) move into the new arch hooks.
>>>>>>> That'll effectively mean to simply drop the Arm stubs afaict (and I
>>>>>>> notice there's some dead code there on x86, which I guess I'll make
>>>>>>> a patch to clean up). But first of all this suggests that you want
>>>>>>> to call the hooks with base page and order, putting the loops there.
>>>>>>
>>>>>> I see your point and agree ... However I see the on-list patches that
>>>>>> remove common memguard_* invocations and x86 bits.
>>>>>> So I assume, this request is not actual anymore, or I still need 
>>>>>> to pass
>>>>>> an order to new arch hooks? Please clarify.
>>>>>
>>>>> Well, that patch (really just the Arm one) effectively takes care of
>>>>> part of what I did say above. Irrespective I continue to think that
>>>>> the hook should take a (page,order) tuple instead of getting invoked
>>>>> once for every order-0 page. And the hook invocations should be placed
>>>>> such that they could fulfill the (being removed) memguard function
>>>>> (iirc that was already the case, at least mostly).
>>>>
>>>> IIUC your suggestion, with your approach, alloc_xenheap_pages() would
>>>> look like:
>>>>
>>>>        for ( i = 0; i < (1u << order); i++ )
>>>>            pg[i].count_info |= PGC_xen_heap;
>>>>
>>>>        arch_alloc_xenheap_pages(pg, 1U << order);
>>>
>>> Like Oleksandr said, the 2nd argument would be just "order".
>>>
>>>> The Arm implementation for arch_alloc_xenheap_pages() would also 
>>>> contain
>>>> a loop.
>>>>
>>>> This could turn out to be quite expensive with large allocation (1GB
>>>> allocation would require 16MB of cache) because the cache may not have
>>>> enough space contain all the pages of that range. So you would have to
>>>> pull twice the page_info in the cache.
>>>
>>> Hmm, that's a fair point. I assume you realize that a similar issue of
>>> higher overhead would occur when using your approach, and when some
>>> memguard-like thing was to reappear: Such mapping operations typically
>>> are more efficient when done on a larger range.
>>
>> Yes, I was aware of that when I wrote my message. However, they are 
>> not necessary at the moment. So I think we can defer the discussion.
>>
>>>  Since that's only a
>>> hypothetical use at this point, I'm willing to accept your preference.
>>> I'd like us to consider one more aspect though: All you need on Arm is
>>> the setting of the exact same bits to the exact same pattern for every
>>> struct page_info involved. Can't we simply have an arch hook returning
>>> that pattern, for generic code to then OR it in alongside PGC_xen_heap?
>>
>> arch_alloc_xenheap_pages() will modify inuse.type_info so we can't or 
>> the value to PGC_xen_heap.
> 
> I wonder, can we apply pattern here at alloc_heap_pages() when 
> initializing type_info?
> https://xenbits.xen.org/gitweb/?p=xen.git;f=xen/common/page_alloc.c;hb=refs/heads/master#l1027
> If yes, the next question would be what indicator to use here to make 
> sure that page is really xenheap page.

Technically that would seem to be possible, by way of passing yet another
argument into alloc_heap_pages(). I'm not (yet) convinced, though, of this
being desirable.

> I also wonder, can we apply pattern for all type of pages here (without 
> differentiating)?

I'm afraid I don't understand this part: How could we get along without
differentiating Xen heap and domain heap pages?

Jan