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Re: [Qemu-devel] [PATCH v2] hw/arm/virt-acpi - reserve ECAM space as PNP

From: Ard Biesheuvel
Subject: Re: [Qemu-devel] [PATCH v2] hw/arm/virt-acpi - reserve ECAM space as PNP0C02 device
Date: Mon, 16 Jan 2017 21:23:29 +0000

On 16 January 2017 at 21:13, Laszlo Ersek <address@hidden> wrote:
> On 01/16/17 20:31, Ard Biesheuvel wrote:
>> On 16 January 2017 at 18:20, Peter Maydell <address@hidden> wrote:
>>> On 16 January 2017 at 17:30, Ard Biesheuvel <address@hidden> wrote:
>>>> On 16 January 2017 at 17:25, Peter Maydell <address@hidden> wrote:
>>>>> On 13 January 2017 at 17:32, Ard Biesheuvel <address@hidden> wrote:
>>>>>> Linux for arm64 v4.10 and later will complain if the ECAM config space is
>>>>>> not reserved in the ACPI namespace:
>>>>>>   acpi PNP0A08:00: [Firmware Bug]: ECAM area [mem 0x3f000000-0x3fffffff] 
>>>>>> not reserved in ACPI namespace
>>>>>> The rationale is that OSes that don't consume the MCFG table should still
>>>>>> be able to infer that the PCI config space MMIO region is occupied.
>>>>>> So update the ACPI table generation routine to add this reservation.
>>>>>> Signed-off-by: Ard Biesheuvel <address@hidden>
>>>>>> ---
>>>>>>  hw/arm/virt-acpi-build.c | 7 +++++++
>>>>>>  1 file changed, 7 insertions(+)
>>>>>> diff --git a/hw/arm/virt-acpi-build.c b/hw/arm/virt-acpi-build.c
>>>>>> index 085a61117378..50d52f685f68 100644
>>>>>> --- a/hw/arm/virt-acpi-build.c
>>>>>> +++ b/hw/arm/virt-acpi-build.c
>>>>>> @@ -310,6 +310,13 @@ static void acpi_dsdt_add_pci(Aml *scope, const 
>>>>>> MemMapEntry *memmap,
>>>>>>      Aml *dev_rp0 = aml_device("%s", "RP0");
>>>>>>      aml_append(dev_rp0, aml_name_decl("_ADR", aml_int(0)));
>>>>>>      aml_append(dev, dev_rp0);
>>>>>> +
>>>>>> +    Aml *dev_res0 = aml_device("%s", "RES0");
>>>>>> +    aml_append(dev_res0, aml_name_decl("_HID", aml_string("PNP0C02")));
>>>>>> +    crs = aml_resource_template();
>>>>>> +    aml_append(crs, aml_memory32_fixed(base_ecam, size_ecam, 
>>>>>> AML_READ_WRITE));
>>>>>> +    aml_append(dev_res0, aml_name_decl("_CRS", crs));
>>>>>> +    aml_append(dev, dev_res0);
>>>>>>      aml_append(scope, dev);
>>>>>>  }
>>>>> This needs to be controlled via the machine class back-compat
>>>>> machinery in hw/arm/virt.c so that it only happens for virt-2.9
>>>>> and later.
>>>> Why exactly?
>>> Because the "virt-2.8" machine has to present to the guest
>>> exactly what "virt" did as of the QEMU 2.8 release, including
>>> any bugs or missing things we happened to have in our ACPI
>>> tables. This allows cross-version compatibility (including
>>> VM migration). Drew will have a more detailed explanation
>>> if you need it.
>> I suspected as much.
>> But in this case, I am not sure if it is worth the trouble: the
>> generated data is only consumed at boot time by the firmware, and I
>> suppose migration involves freezing a VM, including whatever resident
>> firmware image was used to boot the OS, and so this is unlikely to
>> affect migration.
>> But I will let Drew explain ...
> The PCI Firmware Specification (rev 3.1) says in 4.1.2. "MCFG Table
> Description": "The resources can *optionally* be returned in [...]
> EFIGetMemoryMap as reserved memory [...]". (Emphasis mine.) Linux seems
> to *insist* on this kind of reservation however.

No, not at the UEFI level but at the ACPI level. Reservations in the
UEFI memory map describe memory not MMIO space

> PNP0C02 is "General ID for reserving resources required by PnP
> motherboard registers. (Not device specific.)", according to
> <http://www.plasma-online.de/english/identify/serial/pnp_id_pnp.html>.
> So what this patch does is reserve a memory area through ACPI,
> practically as an unspecified "platform resource".

This has been discussed at great length on the linux mailing lists


> There's an alternative that's contained entirely in the firmware. You
> can cover the MMCONFIG area in ArmVirtQemu with an EfiReservedMemoryType
> memory map entry (by producing an appropriate memalloc HOB in PEI, or by
> calling the appropriate gDS memory space map functions in DXE). OVMF
> does the former (memalloc HOB).
> In ArmVirtQemu, we grab the MMCONFIG range from "pci-host-ecam-generic",
> from QEMU's DTB. If you don't dislike the idea, we could cover the range
> as well, right in "ArmVirtPkg/Library/FdtPciPcdProducerLib". That lib
> instance already sets the base address PCD, and makes sure that the
> relevant code is executed only once (in whatever driver module the
> library instance was built into). You could call the gDS functions
> mentioned above from that spot. (The library instance is already
> restricted to DXE_DRIVER and UEFI_DRIVER modules.)

In general, I think describing MMIO in the UEFI memory map is not very
useful, and counter to the spec, which mentions that the memory map
describes memory ("however it is used"), not memory *space* (unless
UEFI itself needs to access it to implement runtime services)

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