Re: [Qemu-devel] [PATCH 2/2] hw/pcie: disable IO port fwd by default for pcie-root-port

[Marcel Apfelbaum]

Hi Laszlo,

On 20/09/2017 14:01, Laszlo Ersek wrote:
>   We now have
>
>      if (mem_pref_32_reserve != (uint32_t)-1 &&
>          mem_pref_64_reserve != (uint64_t)-1) {
>          error_setg(errp,
>
> but after introducing the zero value, this is going to be too strict.
> Consider all the possible combinations:
>
> * (-1; -1): default fw behavior, check is OK
>
> * (-1;  0): fw sticks with the default for the first component, picks
>              "no reservation" for the second component, check is OK
>
> * (-1; >0): fw can treat this identically to ( 0; >0) -- see below. This
>              is justified because, while the first component says "use
>              the default", the entire situation of having such a
>              capability is new, so the firmware can introduce new ways
>              for handling it. The check passes, which is good.
>
> * ( 0;  0): check reports error, but the firmware can handle this case
>              fine (no reservation for either component)
>
> * ( 0; >0): check reports error, but the fw can handle this (no
>              reservation for the first component, specific reservation
>              for the second)
>
> * (>0; >0): conflict, check is OK
>
> (I'm skipping the description of the inverted orderings, such as (0;-1),
> they behave similarly.)
>
> So we need to accept 0 as "valid" for either component:
>
>      if (mem_pref_32_reserve > 0 &&
>          mem_pref_32_reserve < (uint32_t)-1 &&
>          mem_pref_64_reserve > 0 &&
>          mem_pref_64_reserve < (uint64_t)-1) {
>          error_setg(errp,

SeaBIOS  will not allow (0,0) values for pref32 and pref64 fields.
More than that, 0 values are not being taken in consideration.

We may say is a bug and fix it. But taking a step back, we need the
hints to reserve *more* mem range, I am not sure 0 values are 
interesting for mem reservation. Also is also only a hint,
the firmware should decide.

Do we have a concrete scenario where would we want to use (0, non-zero)
or (non-zero, 0) ? (ask for 32/64 prefetch)

Our current semantic is: "we give a hint to the Guest Firmware,
we don't decide. However, giving hints to both pref32 and pref64
is wrong".


Not related: Using 0 value to disable IO works just fine!
No need for disable-IO-fwd property :)

Thanks,
Marcel