Re: [Qemu-devel] [PATCH V2 2/4] pci: reserve 64 bit MMIO range for PCI hotplug

[Marcel Apfelbaum <marcel@redhat.com>]

On 05/18/2016 05:42 PM, Michael S. Tsirkin wrote:
> On Wed, May 18, 2016 at 04:31:46PM +0200, Igor Mammedov wrote:
>> On Wed, 18 May 2016 17:12:09 +0300
>> Marcel Apfelbaum <marcel@redhat.com> wrote:
>>
>>> On 05/18/2016 05:11 PM, Michael S. Tsirkin wrote:
>>>> On Wed, May 18, 2016 at 03:59:29PM +0200, Igor Mammedov wrote:
>>>>> On Sun, 15 May 2016 22:23:32 +0300
>>>>> Marcel Apfelbaum <marcel@redhat.com> wrote:
>>>>>
>>>>>> Using the firmware assigned MMIO ranges for 64-bit PCI window
>>>>>> leads to zero space for hot-plugging PCI devices over 4G.
>>>>>>
>>>>>> PC machines can use the whole CPU addressable range after
>>>>>> the space reserved for memory-hotplug.
>>>>>>
>>>>>> Signed-off-by: Marcel Apfelbaum <marcel@redhat.com>
>>>>> that patch also has side effect of unconditionally adding
>>>>> QWordMemory() resource in PCI0._CRS
>>>>> on all machine types with QEMU generated ACPI tables.
>>>>>
>>>>> Have you tested that it won't break boot of legacy OSes
>>>>> (XP, WS2003, old linux with 32bit kernel)?
>>>>
>>>> It's almost sure it break it.
>>>> Maybe you can check _REV in _CRS to work around this for XP.
>>>
>>> I'll try it.
>> but only after you check if just presence of QWord would crash XP,
>> so in case it doesn't crash we would keep _CRS simple static
>> structure.
>>
>> I very vaguely recall that XP ignored QWord in PCI0._CRS,
>> but it was long time ago so it won't hurt to recheck.
>
> I played with different guests (32 and 64 bit)
> at some point.
>
> Generally, windows tends to crash when CRS resources exceed the
> supported limits
> of physical memory (sometimes with weird off by one errors,
> e.g. win7 32 bit seems to survive with a 36 bit pci hole even though
> this means the max address is 2^37-1 which it can't address).
>
> This might depend on CPU as well.

It seems QEMU returns 40-bit as CPU addressable bits, but I might got it wrong.

>
> Which makes me ask: why don't we fix this in BIOS?
> If you want it to allocate a large window, do it.

I don't follow, BIOS can assign resources to PCI devices, but how to
specify a MMIO range for hot-plug? Add it to the CRS of the host-bridge, right?

Thanks,
Marcel

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