Re: [v3][PATCH 07/16] hvmloader/pci: skip reserved ranges

["Chen, Tiejun" <tiejun.chen@intel.com>]

On 2015/6/16 17:40, Jan Beulich wrote:
>>>> On 16.06.15 at 11:29, <tiejun.chen@intel.com> wrote:
>> I'm trying to walk into this direction:
>>
>>       /*
>>        * We'll skip all space overlapping with reserved memory later,
>>        * so we need to increase mmio_total to compensate them.
>>        */
>>       for ( j = 0; j < memory_map.nr_map ; j++ )
>>       {
>>           uint64_t conflict_size = 0;
>>           if ( memory_map.map[j].type != E820_RAM )
>>           {
>>               reserved_start = memory_map.map[j].addr;
>>               reserved_size = memory_map.map[j].size;
>>               reserved_end = reserved_start + reserved_size;
>>               if ( check_overlap(pci_mem_start, pci_mem_end - pci_mem_start,
>>                                  reserved_start, reserved_size) )
>>               {
>>                   /*
>>                    * Calculate how much mmio range conflict with
>>                    * reserved device memory.
>>                    */
>>                   conflict_size += reserved_size;
>>
>>                   /*
>>                    * But we may need to subtract those sizes beyond the
>>                    * pci memory, [pci_mem_start, pci_mem_end].
>>                    */
>>                   if ( reserved_start < pci_mem_start )
>>                       conflict_size -= (pci_mem_start - reserved_start);
>>                   if ( reserved_end > pci_mem_end )
>>                       conflict_size -= (reserved_end - pci_mem_end);
>>               }
>>           }
>>
>>           if ( conflict_size )
>>           {
>>               uint64_t conflict_size = max_t(
>>                       uint64_t, conflict_size, max_bar_sz);
>>               conflict_size &= ~(conflict_size - 1);
>>               mmio_total += conflict_size;
>>           }
>>       }
>
> This last thing goes in the right direction, but is complete overkill
> when you have a small reserved region and a huge BAR. You

Yeah, this may waste some spaces in this worst case but I this think 
this can guarantee our change don't impact on the original expectation, 
right?

> ought to work out the smallest power-of-2 region enclosing the

Okay. I remember the smallest size of a given PCI I/O space is 8 bytes, 
and the smallest size of a PCI memory space is 16 bytes. So

/* At least 16 bytes to align a PCI BAR size. */
uint64_t align = 16;

reserved_start = memory_map.map[j].addr;
reserved_size = memory_map.map[j].size;

reserved_start = (reserved_star + align) & ~(align - 1);
reserved_size = (reserved_size + align) & ~(align - 1);

Is this correct?

> reserved range (albeit there are tricky corner cases to consider).
>

Yeah, its a little tricky since RMRR always owns a fixed start address, 
so we can't reorder them with all pci bars. I just think at least we 
should provide a correct solution now, then further look into what can 
be optimized. So I think we'd better get conflict_size with 
max(conflict_size, max_bar_sz), right?

Thanks
Tiejun