linux-kernel - Re: [PATCH 2/2] tpm/tpm_ftpm_tee: use send

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Message-ID: <Z-VM56u1zWQSPemh@kernel.org>
Date: Thu, 27 Mar 2025 15:04:39 +0200
From: Jarkko Sakkinen <jarkko@...nel.org>
To: Stefano Garzarella <sgarzare@...hat.com>
Cc: Jason Gunthorpe <jgg@...pe.ca>, Sumit Garg <sumit.garg@...nel.org>,
	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>
Subject: Re: [PATCH 2/2] tpm/tpm_ftpm_tee: use send_recv() op

On Thu, Mar 27, 2025 at 10:27:48AM +0100, Stefano Garzarella wrote:
> On Wed, Mar 26, 2025 at 10:37:09PM +0200, Jarkko Sakkinen wrote:
> > On Wed, Mar 26, 2025 at 05:58:33PM +0200, Jarkko Sakkinen wrote:
> > > On Wed, Mar 26, 2025 at 04:57:47PM +0200, Jarkko Sakkinen wrote:
> > > > On Wed, Mar 26, 2025 at 11:34:01AM -0300, Jason Gunthorpe wrote:
> > > > > On Wed, Mar 26, 2025 at 02:11:12PM +0200, Jarkko Sakkinen wrote:
> > > > >
> > > > > > Generally speaking I don't see enough value in complicating
> > > > > > callback interface. It's better to handle complications in
> > > > > > the leaves (i.e. dictatorship of majority ;-) ).
> > > > >
> > > > > That is very much not the way most driver subsystems view the
> > > > > world. We want to pull logical things into the core code and remove
> > > > > them from drivers to make the drivers simpler and more robust.
> > > > >
> > > > > The amount of really dumb driver boiler plate that this series
> > > > > obviously removes is exactly the sort of stuff we should be fixing by
> > > > > improving the core code.
> > > > >
> > > > > The callback interface was never really sanely designed, it was just
> > > > > built around the idea of pulling the timout processing into the core
> > > > > code for TIS hardware. It should be revised to properly match these
> > > > > new HW types that don't have this kind of timeout mechanism.
> > > >
> > > > Both TIS and CRB, which are TCG standards and they span to many
> > > > different types of drivers and busses. I don't have the figures but
> > > > probably they cover vast majority of the hardware.
> > > >
> > > > We are talking about 39 lines of reduced complexity at the cost
> > > > of complicating branching at the top level. I doubt that there
> > > > is either any throughput or latency issues.
> > > >
> > > > What is measurable benefit? The rationale is way way too abstract
> > > > for me to cope, sorry.
> > > 
> > > E.g., here's how you can get rid of extra cruft in tpm_ftpm_tee w/o
> > > any new callbacks.
> 
> Yeah, I agree that your patch should go in any case, with send_recv() or
> not. It's a good cleanup.
> 
> > 
> > Measurable benefit: no need to allocate memory buffer.
> 
> That's right, I read the whole thread before responding, but that's exactly
> what I wanted to highlight. Implementing send_recv() we could completely
> remove the buffer for the cache here in tpm_ftpm_tee, simplifying it quite a
> bit.
> 
> In tpm_svsm instead we allocate it while probing anyway to avoid having to
> allocate it every time, but we could potentially do the same (I don't know
> if it makes sense honestly). We do this because for SVSM any buffer is fine,
> as it can access all guest kernel memory, whereas IIUC for ftpm it has to be
> taken from shared memory.
> 
> > 
> > Let's take that as a starting point ;-)
> 
> Yeah!
> 
> > 
> > On that basis I can consider this (i.e. something to measure).
> 
> Okay, I explain this better in the commit description for the next version!

Awesome :-) Thanks for the patience with this. I rather precautionaly
throw sticks on the road than go through fixes post upstreaming, which
is factors more nasty and time-consuming...

> 
> Thanks,
> Stefano
> 

BR, Jarkko