Re: [Xen-devel] [PATCH v7] x86/p2m: use large pages for MMIO mappings

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 09/02/16 11:48, Jan Beulich wrote:
>>>> On 09.02.16 at 11:56, <george.dunlap@xxxxxxxxxx> wrote:
>> On 09/02/16 08:42, Jan Beulich wrote:
>>>>>> On 08.02.16 at 19:04, <george.dunlap@xxxxxxxxxx> wrote:
>>>> First -- and this isn't a blocker, but just a question (and sorry if
>>>> it's been answered before) -- why have the "0 means I did it all, <nr
>>>> means I did it partially"?  Why not just return the number of entries
>>>> actually handled?
>>>
>>> Because I view zero as the conventional "success" indicator. No
>>> other deeper reason.
>>
>> For certain classes of functionality, yes.  But for other classes --
>> where the caller will request N but the OS / hypervisor / whatever may
>> do some other number fewer than N, then the convention is to always pass
>> the number of items completed.  read() and write() system calls are like
>> this, as are most of the XENMEM operations (e.g.,
>> XENMEM_increase_resevration).
>>
>> Since this domctl looks a lot like some of those XENMEM operations,
>> wouldn't it make more sense to follow that convention, and return the
>> number of extents actually allocated?
> 
> Well, if comparing with the XENMEM ones, perhaps. But if comparing
> with other domctls, perhaps rather not? I've really been undecided
> here from the very beginning, since I had considered that alternative
> right away.

Well from a brief survey of things that can partially succeed
(getmemlist, gethvmcontext, {get,set}_vcpu_msrs), it seems that most of
them:

1. Use the return value *only* for success or failure; they either
return 0 or -errno, even if they only partially succeed
2. Return the actual number of things accomplished in the struct itself.

You seem to be introducing a new model, where you use the return value
sort of for all three.  (Are there any other hypercalls that behave this
way?)

I don't think sometimes returning the number of things you did and
sometimes returning zero makes any sense.  My suggestion would be either
make "nr_mfns" bidirectional (as similar fields are in the other
domctls) and return 0 on either full or partial success, or just return
the number of mfns actually mapped either on full or partial success.

>> Under those conditions, it looks to me like the following will happen:
>> 1. map_mmio_regions() will call set_mmio_p2m_entry() with order 9
>> 2. set_mmio_p2m_entry() will return 9 (requesting using order 8)
>> 3. map_mmio_regions() will call set_mmio_p2m_entry() with order 8
>> 4. set_mmio_p2m_entry() will call set_typed_p2m_entry() with order 8
>> 5. set_typed_p2m_entry() will read the current p2m entry of >= 9 and
>> continue (since 8 <= 9)
>> 6. set_typed_p2m_entry() will call p2m_set_entry() with an order of 8
>> 7. ept_set_entry() will return -EINVAL, since order % EPT_TABLE_ORDER != 0.
>>
>> Am I missing something?
> 
> Between step 6 and step 7 there is actually p2m_set_entry()
> breaking up the request into chunks or orders the hardware
> supports.

Oh, right -- sorry, I assumed that p2m_set_entry() was just an alias for
the underlying *set_entry() function.  I see now there's a loop handling
arbitrary orders.  Thanks. :-)

 -George


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