Re: [Xen-devel] [V9 PATCH 6/8] pvh dom0: Add and remove foreign pages

[Andres Lagar-Cavilla <andreslc@xxxxxxxxxxxxxx>]

On Apr 23, 2014, at 5:03 AM, "Jan Beulich" <JBeulich@xxxxxxxx> wrote:

>>>> On 23.04.14 at 02:28, <mukesh.rathor@xxxxxxxxxx> wrote:
>> On Tue, 22 Apr 2014 08:28:45 +0100
>> "Jan Beulich" <JBeulich@xxxxxxxx> wrote:
>> 
>>>>>> On 22.04.14 at 02:19, <mukesh.rathor@xxxxxxxxxx> wrote:
>>>> On Wed, 16 Apr 2014 18:37:42 -0700
>>>> Mukesh Rathor <mukesh.rathor@xxxxxxxxxx> wrote:
>>>> 
>>>>> On Wed, 16 Apr 2014 17:00:35 +0100
>>>>> "Jan Beulich" <JBeulich@xxxxxxxx> wrote:
>>>>> 
>> ............
>> 
>>>>     /* following will take a refcnt on the mfn */
>>>>     page = get_page_from_gfn(fdom, fgfn, &p2mt, P2M_ALLOC);
>>>> -    if ( !page || !p2m_is_valid(p2mt) )
>>>> +    if ( !page || !p2m_is_strict_ram(p2mt) )
>>>>     {
>>>>         if ( page )
>>>>             put_page(page);
>>>> diff --git a/xen/include/asm-x86/p2m.h b/xen/include/asm-x86/p2m.h
>>>> index 47604da..e0bf6dc 100644
>>>> --- a/xen/include/asm-x86/p2m.h
>>>> +++ b/xen/include/asm-x86/p2m.h
>>>> @@ -165,6 +165,7 @@ typedef unsigned int p2m_query_t;
>>>> 
>>>> /* Useful predicates */
>>>> +#define p2m_is_strict_ram(_t) (p2m_to_mask(_t) &
>>>> p2m_to_mask(p2m_ram_rw))
>>> 
>>> Not sure - this now precludes mapping log-dirty or r/o pages, i.e. I
>>> think you went from too relaxed to too strict. I think the question
>>> needs answering in a more abstract fashion first, i.e. without code
>>> written immediately: Which pages absolutely need to be foreign-
>>> mappable and which ones can safely get foreign-mapped. My first
>>> thought would be P2M_RAM_TYPES & ~P2M_HOLE_TYPES, with
>>> P2M_HOLE_TYPES contained in P2M_RAM_TYPES generating a kind
>>> of -EAGAIN notification while triggering the re-creation of the page
>>> (which already is an indication that instead of P2M_HOLE_TYPES it
>>> might well need to be P2M_PAGING_TYPES).
>> 
>> The current use case is p2m_ram_rw, my approach would be start with 
>> minimum and add more as needed when features like migration, passthru, etc
>> are added. My understanding of the various p2m types is limited, so I
>> am at your mercy here to advise what types to anticipate. IMO, just
>> checking for P2M_RAM_TYPES might suffice, I can additionally add check
>> to make sure it's not P2M_HOLE_TYPES. Please lmk.
> 
> It's clear that p2m_ram_rw is too narrow: p2m_ram_ro and
> p2m_ram_logdirty will minimally also need accepting here. As for the
> paging/paged and shared types, I have to defer to the respective
> maintainers - Andres, Tim?
Not entirely sure on the context, Hence:

If you need types that represent p2m backed by actual host pages with 
up-to-date contents, although possibly with limited permissions:
_ram_rw | _ram_ro | _ram_logdiry | _ram_paging_out | _ram_shared

If you need types that represent p2m that after fault resolution will be backed 
by actual host pages with up-to-date contents, add:
_populate_on_demand | _ram_paged | _ram_paging_in

Consider also the _grant_map_r[w|o] types (they also fall under the first 
category).

Andres

> 
> Jan
> 


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