Re: HVMlite ABI specification DRAFT B + implementation outline

["Roger Pau Monné" <roger.pau@citrix.com>]

El 10/2/16 a les 13:53, Jan Beulich ha escrit:
>>>> On 10.02.16 at 13:01, <roger.pau@citrix.com> wrote:
>> El 9/2/16 a les 14:24, Jan Beulich ha escrit:
>>>>>> On 08.02.16 at 20:03, <roger.pau@citrix.com> wrote:
>>>>  * 2. HVMlite with (or capable to) PCI-passthrough
>>>>    -----------------------------------------------
>>>>    The current model of PCI-passthrought in PV guests is complex and requires
>>>>    heavy modifications to the guest OS. Going forward we would like to remove
>>>>    this limitation, by providing an interface that's the same as found on bare
>>>>    metal. In order to do this, at least an emulated local APIC should be
>>>>    provided to guests, together with the access to a PCI-Root complex.
>>>>    As said in the 'Hardware description' section above, this will also require
>>>>    ACPI. So this proposed scenario will require the following elements that are
>>>>    not present in the minimal (or default) HVMlite implementation: ACPI, local
>>>>    APIC, IO APIC (optional) and PCI-Root complex.
>>>
>>> Are you reasonably convinced that the absence of an IO-APIC
>>> won't, with LAPICs present, cause more confusion than aid to the
>>> OSes wanting to adopt PVHv2?
>>
>> As long as the data provided in the MADT represent the container
>> provided I think we should be fine. In the case of no IO APICs no
>> entries of type 1 (IO APIC) will be provided in the MADT.
> 
> I understand that, but are certain OSes are prepared for that?

Well, given that some modifications will always be needed in order to
run as a PVH guest, I don't think this should stop us. FWIW, I think
FreeBSD should be able to cope with this without much fuss:

https://svnweb.freebsd.org/base/head/sys/x86/acpica/madt.c?revision=291686&view=co

The detection of local APIC and IO APICs is quite isolated from each
other, and the failure to find any IO APICs should not prevent local
APICs from being enabled. Although I don't think there's any hardware
with this setup, such configuration would be valid from an ACPI point of
view. I've of course not tested this in any way, so those are only
observations by a quick look at the code. I cannot speak about Linux.

Also, thanks for the observations and comments on the document.

Roger.