Re: [PATCH V2] Add a new PIIX option to control PCI hot unplugging of devices on non-root buses

[Ani Sinha]

> On Apr 29, 2020, at 3:45 PM, Michael S. Tsirkin <address@hidden> wrote:
> 
> On Wed, Apr 29, 2020 at 09:14:26AM +0000, Ani Sinha wrote:
>> 
>> 
>>> On Apr 29, 2020, at 2:26 PM, Michael S. Tsirkin <address@hidden> wrote:
>>> 
>>> Even if it seems to work for guests now, if we don't stick to emulating
>>> capabilities that hardware interfaces provide we can never be sure it
>>> will keep working.
>> 
>> OS es use ACPI for PCI bridges: 
>> https://urldefense.proofpoint.com/v2/url?u=https-3A__github.com_qemu_qemu_blob_master_docs_pcie.txt&d=DwIFaQ&c=s883GpUCOChKOHiocYtGcg&r=IIUxIyRwG4RGy57y2nvMNYcDkqW-NHozZ2R38VYcg5U&m=9AnuR62iRsUnmV9ggkS_lqen67etHeCjATFLwCJWxQs&s=As__N9BXf0DTSZkD4cxaQsTeg0exh5GSPqkFSh7fxRk&e=
>>  
>> They use “_EJ0” to detect jot unplug capability: 
>> https://urldefense.proofpoint.com/v2/url?u=https-3A__uefi.org_sites_default_files_resources_ACPI-5F3.pdf&d=DwIFaQ&c=s883GpUCOChKOHiocYtGcg&r=IIUxIyRwG4RGy57y2nvMNYcDkqW-NHozZ2R38VYcg5U&m=9AnuR62iRsUnmV9ggkS_lqen67etHeCjATFLwCJWxQs&s=SDgOhKGpnlzfbvAOFl2m9a-tdPZEUcCwKWsgkVDafyM&e=
>>  
>> 
>> 6.3.3 _EJx (Eject) These control methods are optional and are supplied for 
>> devices that support a software-controlled VCRstyle ejection mechanism or 
>> that require an action be performed such as isolation of power/data lines 
>> before the device can be removed from the system. To support warm (system is 
>> in a sleep state) and hot (system is in S0) removal, an _EJx control method 
>> is listed for each sleep state from which the device supports removal, where 
>> x is the sleeping state supported. For example, _EJ0 indicates the device 
>> supports hot removal; _EJ1–EJ4 indicate the device supports warm removal.
> 
> Yes. So if there's no _EJx then it's reasonable to assume you can't
> isolate the slot, and so no hot-plug will happen either.

Where are you getting that? This isn’t true. “_SUN” is used to detect the slot 
unique number. Windows does allow hot plugging on the bridge on which EJ0 has 
been turned off.

> 
>> 
>> Note that “these control methods are optional” line. If the OS adheres to 
>> the spec, it should not expect them to exist all the time.
>> 
> 
> 
> 
> -- 
> MST
>