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Re: [Qemu-devel] [PATCH v8 2/5] tpm: implement virtual memory device for

From: Laszlo Ersek
Subject: Re: [Qemu-devel] [PATCH v8 2/5] tpm: implement virtual memory device for TPM PPI
Date: Tue, 17 Jul 2018 15:04:25 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.7.0

On 07/17/18 12:39, Marc-André Lureau wrote:
> Hi
> On Tue, Jul 17, 2018 at 12:03 PM, Igor Mammedov <address@hidden> wrote:
>> On Mon, 16 Jul 2018 14:59:45 +0200
>> Marc-André Lureau <address@hidden> wrote:
>>> From: Stefan Berger <address@hidden>
>>> Implement a virtual memory device for the TPM Physical Presence interface.
>>> The memory is located at 0xFED45000 and used by ACPI to send messages to the
>>> firmware (BIOS) and by the firmware to provide parameters for each one of
>>> the supported codes.
>> ...
>> another question wrt E820 table, is firmware putting a reservation record
>> for this RAM area overlay or should we do it QEMU?

What OS requires this memmap entry?

> There doesn't seem to be any reservation done in ovmf, or on my
> laptop.

Relevant point, about your laptop.

> I also wonder if it should be done.

IMO: no, it shouldn't.

> And how OS deal with that today (/proc/iomem is correct).
> Laszlo, any idea?

In my experience, exposing such MMIO areas that belong to *platform*
devices, in the UEFI memory map, is an inconsistent practice. I can list
at least four different cases:

(1) A UEFI runtime driver is using the MMIO area for providing a UEFI
runtime service. In this case, the firmware must expose the area as
"MMIO" type memory, and set the "runtime" attribute on it. In response,
the OS will map the area with virtual addresses, directly in response to
the memmap entry.

(2) A firmware SMM driver is using the MMIO area for providing a
privileged part of a UEFI runtime service. Because SMM uses its own page
table anyway (protected from the OS), advertizing the area in the UEFI
memmap is unneeded. It can simply be left as a gap.

(3) Linux requires the firmware to cover every MMCONFIG area (each of
which belongs to a PCIe host bridge, which is a platform device) with a
"reserved" type memmap entry. To me this looks like an outlier, although
I *very* vaguely recall that some spec recommended this.

Ah, right, the PCI Firmware Specification says, under "MCFG Table
Description": "*If* the operating system does not natively comprehend
reserving the MMCFG region, the MMCFG region must be reserved by
firmware." (Emphasis mine.) In other words, this is a provision in the
PCI firmware spec so that platform firmware paper over the fact that the
ACPI subsystem in the OS kernel is limited. Linux, given that it
understand MCFG just fine, totally should not *require* the firmware to
reserve this region in the UEFI memory map as well. Anyway, Linux does
require it (incorrectly, in my opinion), so OVMF complies.

(4) To my understanding, unless given Linux or Windows quirks like (3),
or UEFI runtime drivers like (1), we shouldn't expose, in the UEFI
memmap, MMIO areas related to platform devices. In particular, the
"MMIO" type memory *without* the "runtime" attribute (see (1)) should
never be used in the UEFI memmap (and edk2 will never create such
entries) -- the UEFI spec says that all such areas (resources) should be
communicated through ACPI tables ("All system memory-mapped IO
information should come from ACPI tables" -- see EfiMemoryMappedIO in
"Table 26. Memory Type Usage after ExitBootServices()").

In brief, the TPM PPI MMIO area should just be left as a gap in the UEFI
memmap. (Case (4) applies.)


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