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


From: Laszlo Ersek
Subject: Re: [Qemu-devel] [PATCH v2] Add ACPI tables for TPM
Date: Wed, 30 Jul 2014 17:37:26 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.6.0

On 07/30/14 17:10, Stefan Berger wrote:
> Laszlo Ersek <address@hidden> wrote on 07/30/2014 10:36:38 AM:
> 
>> From: Laszlo Ersek <address@hidden>
>> To: "Michael S. Tsirkin" <address@hidden>, Stefan 
>> Berger/Watson/address@hidden
>> Cc: address@hidden, Stefan Berger <address@hidden>
>> Date: 07/30/2014 10:36 AM
>> Subject: Re: [PATCH v2] Add ACPI tables for TPM
>>
>> 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.
> 
> This is the code for what I would call 'pointer relocation'. The TCPA
> table is not the only place where this is used, but why is it an issue
> there while not with the following?
> 
>     fadt->firmware_ctrl = cpu_to_le32(facs);
>     /* FACS address to be filled by Guest linker */
>     bios_linker_loader_add_pointer(linker, ACPI_BUILD_TABLE_FILE,  
>                                    ACPI_BUILD_TABLE_FILE,
>                                    table_data, &fadt->firmware_ctrl,
>                                    sizeof fadt->firmware_ctrl);

(Responding wrt. to OVMF:) because EFI_ACPI_TABLE_PROTOCOL special cases
the installation of FACS. This is visible both in the UEFI
specification, and in the edk2 implementation.

That is, OVMF can avoid relocating the "fadt->firmware_ctrl" pointer,
because edk2's EFI_ACPI_TABLE_PROTOCOL will do that automatically, when
OVMF requests the installation of FACS.

No such special case exists for the installation of the TCG event log
area (which is by itself not even an ACPI table as I gather). *If* the
TCG event log were an ACPI table in its own right, *and* edk2's
EFI_ACPI_TABLE_PROTOCOL implementation had a special case for it, then
when OVMF installed the event log, EFI_ACPI_TABLE_PROTOCOL would update
TCPA.LASA to it.

Thanks
Laszlo



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