Re: [Qemu-devel] [PATCH v2] Add ACPI tables for TPM

[Laszlo Ersek]

On 07/30/14 15:20, Michael S. Tsirkin wrote:
> On Tue, Jul 29, 2014 at 06:52:19AM -0400, Stefan Berger wrote:
>> From: Stefan Berger <address@hidden>
>>
>> Add an SSDT ACPI table for the TPM device.
>> Add a TCPA table for BIOS logging area when a TPM is being used.
>>
>> The latter follows this spec here:
>>
>> http://www.trustedcomputinggroup.org/files/static_page_files/DCD4188E-1A4B-B294-D050A155FB6F7385/TCG_ACPIGeneralSpecification_PublicReview.pdf

(Thanks for CC'ing me, Michael.)

I skimmed this spec.

>> +static void
>> +build_tpm_tcpa(GArray *table_data, GArray *linker)
>> +{
>> +    Acpi20Tcpa *tcpa;
>> +    uint32_t log_area_minimum_length = TPM_LOG_AREA_MINIMUM_SIZE;
>> +    uint64_t log_area_start_address;
>> +    size_t len = log_area_minimum_length + sizeof(*tcpa);
>> +
>> +    log_area_start_address = table_data->len + sizeof(*tcpa);
>> +
>> +    tcpa = acpi_data_push(table_data, len);
>> +
>> +    tcpa->platform_class = cpu_to_le16(TPM_TCPA_ACPI_CLASS_CLIENT);
>> +    tcpa->log_area_minimum_length = cpu_to_le32(log_area_minimum_length);
>> +    tcpa->log_area_start_address = cpu_to_le64(log_area_start_address);
>> +
>> +    /* LASA address to be filled by Guest linker */
> 
> Hmm, you are simply allocating log area as part of the ACPI table.  It
> works because bios happens to allocate tables from high memory.
> But I think this is a problem in practice because
> bios is allowed to allocate acpi memory differently.
> On the other hand log presumably needs to reside in
> physical memory somewhere.
> 
> If you need bios to allocate this memory, then we will
> need a new allocation type for this, add it to linker
> in bios and qemu.
> 
> Alternatively, find some other way to get hold of
> physical memory.
> Is there a way to disable the log completely?
> As defined in your patch, I doubt there's anything there, ever ..
> 
> 
> 
>> +    bios_linker_loader_add_pointer(linker, ACPI_BUILD_TABLE_FILE,
>> +                                   ACPI_BUILD_TABLE_FILE,
>> +                                   table_data, 
>> &tcpa->log_area_start_address,
>> +                                   sizeof(tcpa->log_area_start_address));
>> +    build_header(linker, table_data,
>> +                 (void *)tcpa, "TCPA", sizeof(*tcpa), 2);
>> +}

So here's my understanding. The spec referenced above describes three
ACPI tables: two (client vs. server) for TPM 1.2, and a third one
(usable by both client & server platforms) for TPM 2.0.

The code above prepares a TPM 1.2 table. (Signature: "TCPA".)

This table has a field called LASA (Log Area Start Address) which points
to somewhere in (guest-)physical memory. The patch adds a "dummy range"
to the end of the TCPA table itself, and asks the linker to set LASA to
the beginning of that range.

This won't work in OVMF, and not just because of the reason that Michael
mentions (ie. because the firmware, in particular SeaBIOS, might
allocate the TCPA table in an area that is unsuitable as LASA target).

Rather, in OVMF this won't work because OVMF doesn't implement the
linking part of the linker. The *generic* edk2 protocol
(EFI_ACPI_TABLE_PROTOCOL, which is coded outside of OVMF) that OVMF uses
(as a client) to install ACPI tables in guest-phys memory requires
tables to be passed in one-by-one.

The EFI_ACPI_TABLE_PROTOCOL implementation in edk2 handles *some*
well-known tables specially. It has knowledge of their internal
pointers, and when you install an ACPI table, EFI_ACPI_TABLE_PROTOCOL
updates pointers automatically. (For example when you install the FACS,
the protocol links it automatically into FACP.)

The EFI_ACPI_TABLE_PROTOCOL implementation in edk2 doesn't seem to know
anything about the TCPA table, let alone the unstructured (?) TCG event
log that is pointed-to by TCPA.LASA.

(I grepped for the TCPA signature,
EFI_ACPI_5_0_TRUSTED_COMPUTING_PLATFORM_ALLIANCE_CAPABILITIES_TABLE_SIGNATURE.)

This means that if you pass down a TCPA table, OVMF will install it
right now, but TCPA.LASA will be bogus.

If I wanted to implement the complete linker as Michael envisioned it,
then I'd have to avoid edk2's EFI_ACPI_TABLE_PROTOCOL, and implement
ACPI table installation from zero, trying to mimic the SeaBIOS client
code, but in a way that matches the UEFI environment. I'm not ready to
do that. Definitely not without an "official" human-language
specification of the linker-loader interface.

I skimmed the patch but I'm not sure what exactly the TPM emulation in
qemu depends on. Is it a command line option? Is it default for some
machine types?

Alternatively, I could recognize the TCPA signature in OVMF when parsing
the ACPI blobs for table headers, and filter it out.

Thanks
Laszlo