Re: [Qemu-devel] [PATCH v4 5/5] docs: update documentation considering PCIE-PCI bridge

[Laszlo Ersek]

On 08/08/17 21:21, Aleksandr Bezzubikov wrote:
> 2017-08-08 18:11 GMT+03:00 Laszlo Ersek <address@hidden>:
>> one comment below
>>
>> On 08/05/17 22:27, Aleksandr Bezzubikov wrote:
>>
>>> +Capability layout (defined in include/hw/pci/pci_bridge.h):
>>> +
>>> +    uint8_t id;     Standard PCI capability header field
>>> +    uint8_t next;   Standard PCI capability header field
>>> +    uint8_t len;    Standard PCI vendor-specific capability header field
>>> +
>>> +    uint8_t type;   Red Hat vendor-specific capability type
>>> +                    List of currently existing types:
>>> +                        QEMU_RESERVE = 1
>>> +
>>> +
>>> +    uint32_t bus_res;   Minimum number of buses to reserve
>>> +
>>> +    uint64_t io;        IO space to reserve
>>> +    uint64_t mem        Non-prefetchable memory to reserve
>>> +    uint64_t mem_pref;  Prefetchable memory to reserve
>>
>> (I apologize if I missed any concrete points from the past messages
>> regarding this structure.)
>>
>> How is the firmware supposed to know whether the prefetchable MMIO
>> reservation should be made in 32-bit or 64-bit address space? If we
>> reserve prefetchable MMIO outside of the 32-bit address space, then
>> hot-plugging a device without 64-bit MMIO support could fail.
>>
>> My earlier request, to distinguish "prefetchable_32" from
>> "prefetchable_64" (mutually exclusively), was so that firmware would
>> know whether to restrict the MMIO reservation to 32-bit address
>> space.
>
> IIUC now (in SeaBIOS at least) we just assign this PREF registers
> unconditionally,
> so the decision about the mode can be made basing on !=0
> UPPER_PREF_LIMIT register.
> My idea was the same - we can just check if the value doesn't fit into
> 16-bit (PREF_LIMIT reg size, 32-bit MMIO). Do we really need separate
> fields for that?

The PciBusDxe driver in edk2 tracks 32-bit and 64-bit MMIO resources
separately from each other, and other (independent) logic exists in it
that, on some conditions, allocates 64-bit MMIO BARs from 32-bit address
space. This is just to say that the distinction is intentional in
PciBusDxe.

Furthermore, the Platform Init spec v1.6 says the following (this is
what OVMF will have to comply with, in the "platform hook" called by
PciBusDxe):

> 12.6 PCI Hot Plug PCI Initialization Protocol
> EFI_PCI_HOT_PLUG_INIT_PROTOCOL.GetResourcePadding()
> ...
> Padding  The amount of resource padding that is required by the PCI
>          bus under the control of the specified HPC. Because the
>          caller does not know the size of this buffer, this buffer is
>          allocated by the callee and freed by the caller.
> ...
> The padding is returned in the form of ACPI (2.0 & 3.0) resource
> descriptors. The exact definition of each of the fields is the same as
> in the
> EFI_PCI_HOST_BRIDGE_RESOURCE_ALLOCATION_PROTOCOL.SubmitResources()
> function. See the section 10.8 for the definition of this function.

Following that pointer:

> 10.8 PCI HostBridge Code Definitions
> 10.8.2 PCI Host Bridge Resource Allocation Protocol
>
> Table 8. ACPI 2.0 & 3.0 QWORD Address Space Descriptor Usage
>
> Byte    Byte    Data  Description
> Offset  Length
> ...
> 0x03    0x01          Resource type:
>                         0: Memory range
>                         1: I/O range
>                         2: Bus number range
> ...
> 0x05    0x01          Type-specific flags. Ignored except as defined
>                       in Table 3-3 and Table 3-4 below.
>
> 0x06    0x08          Address Space Granularity. Used to differentiate
>                       between a 32-bit memory request and a 64-bit
>                       memory request. For a 32-bit memory request,
>                       this field should be set to 32. For a 64-bit
>                       memory request, this field should be set to 64.
>                       Ignored for I/O and bus resource requests.
>                       Ignored during GetProposedResources().

The "Table 3-3" and "Table 3-4" references under "Type-specific flags"
are out of date (spec bug); in reality those are:
- Table 10. I/O Resource Flag (Resource Type = 1) Usage,
- Table 11. Memory Resource Flag (Resource Type = 0) Usage.

The latter is relevant here:

> Table 11. Memory Resource Flag (Resource Type = 0) Usage
>
> Bits      Meaning
> ...
> Bit[2:1]  _MEM. Memory attributes.
>           Value and Meaning:
>             0 The memory is nonprefetchable.
>             1 Invalid.
>             2 Invalid.
>             3 The memory is prefetchable.
>           Note: The interpretation of these bits is somewhat different
>           from the ACPI Specification. According to the ACPI
>           Specification, a value of 0 implies noncacheable memory and
>           the value of 3 indicates prefetchable and cacheable memory.

So whatever OVMF sees in the capability, it must be able to translate to
the above representation.

Thanks
Laszlo

>
>>
>> This is based on an earlier email from Alex to me:
>>
>> On 10/03/16 18:01, Alex Williamson wrote:
>>> I don't think there's such a thing as a 64-bit non-prefetchable
>>> aperture.  In fact, there are not separate 32 and 64 bit
>>> prefetchable apertures.  The apertures are:
>>>
>>> I/O base/limit - (default 16bit, may be 32bit)
>>> Memory base/limit - (32bit only, non-prefetchable)
>>> Prefetchable Memory base/limit - (default 32bit, may be 64bit)
>>>
>>> This is according to Table 3-2 in the PCI-to-PCI bridge spec rev
>>> 1.2.
>>
>> I don't care much about the 16-bit vs. 32-bit IO difference (that's
>> entirely academic and the Platform Spec init doesn't even provide a
>> way for OVMF to express such a difference). However, the optional
>> restriction to 32-bit matters for the prefetchable MMIO aperture.
>>
>> Other than this, the patch looks good to me, and I'm ready to R-b.
>>
>> Thanks!
>> Laszlo