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

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Apr 29, 2020 at 09:14:26AM +0000, Ani Sinha wrote:
> 
> 
> > On Apr 29, 2020, at 2:26 PM, Michael S. Tsirkin <mst@redhat.com> 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://github.com/qemu/qemu/blob/master/docs/pcie.txt
> They use “_EJ0” to detect jot unplug capability: https://uefi.org/sites/default/files/resources/ACPI_3.pdf
> 
> 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.

> 
> 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