Re: [Xen-devel] [PATCH for 4.7] Remove HVMMEM_mmio_write_dm from the public interface.

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 28.04.16 at 13:40, <yu.c.zhang@xxxxxxxxxxxxxxx> wrote:
> On 4/28/2016 6:57 PM, Jan Beulich wrote:
>>>>> On 28.04.16 at 12:42, <george.dunlap@xxxxxxxxxx> wrote:
>>> That might have been slightly cleaner; but we're going to have to put it
>>> back as soon as the development window opens anyway, so I don't really
>>> see the point of going through the effort of respinning the patch again.
>>>
>>> Would you be willing to ack this version anyway?
>>
>> I have no problem doing so (and in fact I have it on my to by
>> committed list already), it is just looked slightly confusing (and
>> I had already typed half a reply that this isn't what was discussed
>> until I properly looked at the next hunk), and hence I wanted to
>> understand the motivation. And btw., I'm not convinced it would
>> need to be put there anyway later: I don't view the used
>> mechanism as a good (read: extensible) one to deal with what
>> would be holes in the array above. Indeed we can't leave them
>> uninitialized (as that would mean p2m_ram_rw), but I think we
>> should better find a way to initialize _all_ unused slots without
>> requiring an initializer for each of them. Sadly the desire to allow
>> compilation with clang prohibits the most natural solution:
>>
>>         static const p2m_type_t memtype[] = {
>>             [0 ... <upper-bound> - 1] = p2m_invalid,
> 
> Not sure if this will compile? Can have a try. :)

gcc will like it, but as said clang won't (afair).

>>             [HVMMEM_ram_rw]  = p2m_ram_rw,
>>             [HVMMEM_ram_ro]  = p2m_ram_ro,
>>             [HVMMEM_mmio_dm] = p2m_mmio_dm,
>>         };
>>
>> Maybe we could do (altering the second hunk of this patch)
>>
>> @@ -5553,7 +5551,10 @@ long do_hvm_op(unsigned long op, 
> XEN_GUEST_HANDLE_PARAM(void) arg)
>>               ((a.first_pfn + a.nr - 1) > domain_get_maximum_gpfn(d)) )
>>              goto setmemtype_fail;
>>
>> -        if ( a.hvmmem_type >= ARRAY_SIZE(memtype) )
>> +        BUILD_BUG_ON(p2m_ram_rw);
>> +        BUILD_BUG_ON(HVMMEM_ram_rw);
>> +        if ( a.hvmmem_type >= ARRAY_SIZE(memtype) ||
>> +             (a.hvmmem_type && !memtype[a.hvmmem_type]) )
> 
> I guess by !memtype[a.hvmmem_type] you are trying to check if it's
> p2m_invalid? But p2m_ram_rw is 0, and p2m_invalid is 1. So may be it
> should be checked like memtype[a.hvmmem_type] < 0 and initialize the
> holes with -1.

No. As said, I want to avoid explicit initializers for unused slots,
and hence it has to be zero that gets checked against.

> But I still wonder is this really necessary? Because we only have one
> hole in this array in the forseeable future.

How do you know?

Jan


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