Re: [Qemu-devel] [PATCH v2 2/5] pci: store PCI hole ranges in guestinfo structure

[Gerd Hoffmann <kraxel@redhat.com>]

On 05/30/13 14:19, Michael S. Tsirkin wrote:
> On Thu, May 30, 2013 at 02:16:13PM +0200, Gerd Hoffmann wrote:
>>   Hi,
>>
>>> +    } else {
>>> +        guest_info->pci_info.w64.begin = 0x100000000ULL + above_4g_mem_size;
>>> +        guest_info->pci_info.w64.end =  guest_info->pci_info.w64.begin +
>>> +            (0x1ULL << 62);
>>
>> Doesn't this give unaligned windows?
> 
> PCI Bridge windows do not need to be size aligned.
> 
> In any case, the windows are *exactly* as calculated
> by seabios - apparently it does not size-align windows either.

Surely not.  SeaBIOS sizes the 64bit window according to the space
needed by the 64bit bars it wants to map there.

>>> +    /* Set PCI window size the way seabios has always done it. */
>>> +    /* TODO: consider just starting at below_4g_mem_size */
>>
>> Used to be that way.  Was changed for alignment reasons (i.e. 1G window
>> starts at 1G border etc).
> 
> Where's the alignment requirement coming from?

seabios creates a mtrr entry for the window, which doesn't work in case
it isn't aligned (at least not with a single entry).

Also real hardware tends to do it this way.

cheers,
  Gerd