Re: [PATCH] Fix PCI hotplug AML

[Paul Durrant <xadimgnik@xxxxxxxxx>]

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.
  Paul

> diff --git a/tools/libacpi/mk_dsdt.c b/tools/libacpi/mk_dsdt.c
> index 1176da80ef..1d27809116 100644
> --- a/tools/libacpi/mk_dsdt.c
> +++ b/tools/libacpi/mk_dsdt.c
> @@ -431,7 +431,7 @@ int main(int argc, char **argv)
>                   stmt("Store", "0x89, \\_GPE.DPT2");
>               }
>               if ( slot & 1 )
> -                stmt("ShiftRight", "0x4, \\_GPE.PH%02X, Local1", slot & ~1);
> +                stmt("ShiftRight", "\\_GPE.PH%02X, 0x04, Local1", slot & ~1);
>               else
>                   stmt("And", "\\_GPE.PH%02X, 0x0f, Local1", slot & ~1);
>               stmt("Return", "Local1"); /* IN status as the _STA */