linux-kernel - Re: [PATCH 1/2] tpm: add send_recv() op in tpm_class

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Message-ID: <wwf3brf4rtdh7ciejgbjesy32ywqxw5vrpuznyee2yp4arrtmw@gspriiauxvgt>
Date: Thu, 27 Mar 2025 15:44:00 +0100
From: Stefano Garzarella <sgarzare@...hat.com>
To: Jarkko Sakkinen <jarkko@...nel.org>
Cc: Jason Gunthorpe <jgg@...pe.ca>, linux-kernel@...r.kernel.org, 
	Peter Huewe <peterhuewe@....de>, linux-integrity@...r.kernel.org, 
	James Bottomley <James.Bottomley@...senpartnership.com>, Jens Wiklander <jens.wiklander@...aro.org>, 
	Sumit Garg <sumit.garg@...nel.org>
Subject: Re: [PATCH 1/2] tpm: add send_recv() op in tpm_class_ops

On Thu, Mar 27, 2025 at 03:02:32PM +0200, Jarkko Sakkinen wrote:
>On Thu, Mar 27, 2025 at 10:48:17AM +0100, Stefano Garzarella wrote:
>> On Wed, Mar 26, 2025 at 06:53:51PM +0200, Jarkko Sakkinen wrote:
>> > On Thu, Mar 20, 2025 at 04:24:32PM +0100, Stefano Garzarella wrote:
>> > > From: Stefano Garzarella <sgarzare@...hat.com>
>> > >
>> > > Some devices do not support interrupts and provide a single operation
>> > > to send the command and receive the response on the same buffer.
>> > >
>> > > To support this scenario, a driver could set TPM_CHIP_FLAG_IRQ in the
>> > > chip's flags to get recv() to be called immediately after send() in
>> > > tpm_try_transmit(), or it needs to implement .status() to return 0,
>> > > and set both .req_complete_mask and .req_complete_val to 0.
>> > >
>> > > In order to simplify these drivers and avoid temporary buffers to be
>> >
>> > Simplification can be addressed with no callback changes:
>> >
>> > https://lore.kernel.org/linux-integrity/20250326161838.123606-1-jarkko@kernel.org/T/#u
>> >
>> > I also noticed that tpm_ftpm_tee initalized req_complete_mask and
>> > req_complete_val explictly while they would be already implicitly
>> > zero.
>> >
>> > So it reduces this just a matter of getting rid off the extra
>> > buffer.
>>
>> Yep, as mentioned I think your patch should go either way. So here I can
>> rephrase and put the emphasis on the temporary buffer and the driver
>> simplification.
>
>Yes. Removing extra copy is a goal that can only make sense!
>
>>
>> >
>> > > used between the .send() and .recv() callbacks, introduce a new callback
>> > > send_recv(). If that callback is defined, it is called in
>> > > tpm_try_transmit() to send the command and receive the response on
>> > > the same buffer in a single call.
>> >
>> > I don't find anything in the commit message addressing buf_len an
>> > cmd_len (vs "just len"). Why two lengths are required?
>> >
>> > Not completely rejecting but this explanation is incomplete.
>>
>> Right.
>>
>> The same buffer is used as input and output.
>> For input, the buffer contains the command (cmd_len) but the driver can use
>> the entire buffer for output (buf_len).
>> It's basically the same as in tpm_try_transmit(), but we avoid having to
>> parse the header in each driver since we already do that in
>> tpm_try_transmit().
>>
>> In summary cmd_len = count = be32_to_cpu(header->length).
>>
>> I admit I'm not good with names, would you prefer a different name or is it
>> okay to explain it better in the commit?
>>
>> My idea is to add this:
>>
>>     `buf` is used as input and output. It contains the command
>>     (`cmd_len` bytes) as input. The driver will be able to use the
>>     entire buffer (`buf_len` bytes) for the response as output.
>>     Passing `cmd_len` is an optimization to avoid having to access the
>>     command header again in each driver and check it.
>
>This makes more sense. Maybe we could name them as buf_size and
>cmd_len to further make dead obvious the use and purpose.

Yeah, I see. I'll do!

>
>>
>> WDYT?
>
>I just want to get this done right if it is done at all, so here's
>one more suggestion:
>
>1. Add TPM_CHIP_FLAG_SYNC
>2. Update send() parameters.

So, IIUC something like this:

     int (*send) (struct tpm_chip *chip, u8 *buf, size_t cmd_len, size_t buf_size);

Where `buf_size` is ignored if the driver doesn't set TPM_CHIP_FLAG_SYNC.

Right?

>
>You don't have to do anything smart with the new parameter other than
>add it to leaf drivers.

Okay, this should answer my question :-) (I leave it just to be sure).

>It makes the first patch bit more involved but
>this way we end up keeping the callback interface as simple as it was.

Yep, I see.
And maybe I need to change something in tpm_try_transmit() because now
send() returns 0 in case of success, but after the change it might
return > 0 in case TPM_CHIP_FLAG_SYNC is set. But I will see how to
handle this.

>
>I'm also thinking that for async case do we actually need all those
>complicated masks etc. or could we simplify that side but it is
>definitely out-of-scope for this patch set (no need to worry about
>this).

I see, I can take a look later in another series.

Thanks,
Stefano