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SubjectRe: [PATCH 2/2] tpm: tpm_crb: enhance resource mapping mechanism for supporting AMD's fTPM
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On Tue, 2019-09-03 at 18:56 +0900, Seunghun Han wrote:
> Thank you for your notification. I am sorry. I missed it and
> misunderstood Jarkko's idea. So, I would like to invite Matthew
> Garrett to this thread and attach my opinion on that. The problem is
> that command and response buffers are in ACPI NVS area. ACPI NVS area
> is saved and restored by drivers/acpi/nvs.c during hibernation, so
> command and response buffers in ACPI NVS are also handled by nvs.c
> file. However, TPM CRB driver uses the buffers to control a TPM
> device, therefore, something may break.
>
> I agree on that point. To remove uncertainty and find the solution,
> I read the threads we discussed and did research about two points, 1)
> the race condition and 2) the unexpected behavior of the TPM device.
>
> 1) The race condition concern comes from unknowing buffer access order
> while hibernation.
> If nvs.c and TPM CRB driver access the buffers concurrently, the race
> condition occurs. Then, we can't know the contents of the buffers
> deterministically, and it may occur the failure of TPM device.
> However, hibernation_snapshot() function calls dpm_suspend() and
> suspend_nvs_save() in order when the system enters into hibernation.
> It also calls suspend_nvs_restore() and dpm_resume() in order when the
> system exits from hibernation. So, no race condition occurs while
> hibernation, and we always guarantee the contents of buffers as we
> expect.
>
> 2) The unexpected behavior of the TPM device.
> If nvs.c saves and restores the contents of the TPM CRB buffers while
> hibernation, it may occur the unexpected behavior of the TPM device
> because the buffers are used to control the TPM device. When the
> system entered into hibernation, suspend_nvs_save() saved the command
> and response buffers, and they had the last command and response data.
> After exiting from hibernation, suspend_nvs_restore() restored the
> last command and response data into the buffers and nothing happened.
> I realized that they were just buffers. If we want to send a command
> to the TPM device, we have to set the CRB_START_INVOKE bit to a
> control_start register of a control area. The control area was not in
> the ACPI NVS area, so it was not affected by nvs.c file. We can
> guarantee the behavior of the TPM device.
>
> Because of these two reasons, I agreed on Jarkko's idea in
> https://lkml.org/lkml/2019/8/29/962 . It seems that removing or
> changing regions described in the ACPI table is not natural after
> setup. In my view, saving and restoring buffers was OK like other NVS
> areas were expected because the buffers were in ACPI NVS area.
>
> So, I made and sent this patch series. I would like to solve this
> AMD's fTPM problem because I have been doing research on TPM and this
> problem is critical for me (as you know fTPM doesn't work). If you
> have any other concern or advice on the patch I made, please let me
> know.

Please take time to edit your responses. Nobody will read that properly
because it is way too exhausting. A long prose only indicates unclear
thoughts in the end. If you know what you are doing, you can put things
into nutshell only in few senteces.

/Jarkko

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