Re: [PATCH] Fix PCI hotplug AML

[Jan Beulich <jbeulich@xxxxxxxx>]

On 20.03.2023 10:04, Paul Durrant wrote:
> On 17/03/2023 10:32, David Woodhouse wrote:
>> From: David Woodhouse <dwmw@xxxxxxxxxxxx>
>>
>> The emulated PIIX3 uses a nybble for the status of each PCI function,
>> so the status for e.g. slot 0 functions 0 and 1 respectively can be
>> read as (\_GPE.PH00 & 0x0F), and (\_GPE.PH00 >> 0x04).
>>
>> The AML that Xen gives to a guest gets the operand order for the odd-
>> numbered functions the wrong way round, returning (0x04 >> \_GPE.PH00)
>> instead.
>>
>> As far as I can tell, this was the wrong way round in Xen from the
>> moment that PCI hotplug was first introduced in commit 83d82e6f35a8:
>>
>> +                    ShiftRight (0x4, \_GPE.PH00, Local1)
>> +                    Return (Local1) /* IN status as the _STA */
>>
>> Or maybe there's bizarre AML operand ordering going on there, like
>> Intel's wrong-way-round assembler, and it only broke later when it was
>> changed to being generated?
>>
>> Either way, it's definitely wrong now, and instrumenting a Linux guest
>> shows that it correctly sees _STA being 0x00 in function 0 of an empty
>> slot, but then the loop in acpiphp_glue.c::get_slot_status() goes on to
>> look at function 1 and sees that _STA evaluates to 0x04. Thus reporting
>> an adapter is present in every slot in /sys/bus/pci/slots/*
>>
>> Quite why Linux wants to look for function 1 being physically present
>> when function 0 isn't... I don't want to think about right now.
>>
>> Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
>> Fixes: 83d82e6f35a8 ("hvmloader: pass-through: multi-function PCI hot-plug")
>> ---
>> Utterly untested in Xen. Tested the same change in a different
>> environment which is using precisely the *same* AML for guest
>> compatibility.
>>
> 
> This AML only relates to the hotplug controller for qemu-trad so it's 
> unlikely anyone particularly cares any more. In fact I'm kind of 
> surprised the generation code still exists.

Why would it not exist anymore? Use of qemu-trad is deprecated and
advised against, but it's still possible to use it. Otherwise quite a
bit of cleanup in libxl could also happen, for example.

Jan