linux-kernel - Re: [PATCH] tpm: Make timeout logic simpler and more robust

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Date:   Tue, 12 Mar 2019 17:39:31 +0200
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     James Bottomley <James.Bottomley@...senPartnership.com>
Cc:     Calvin Owens <calvinowens@...com>, Peter Huewe <peterhuewe@....de>,
        Jason Gunthorpe <jgg@...pe.ca>, Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH] tpm: Make timeout logic simpler and more robust

On Tue, Mar 12, 2019 at 07:42:46AM -0700, James Bottomley wrote:
> On Tue, 2019-03-12 at 14:50 +0200, Jarkko Sakkinen wrote:
> > On Mon, Mar 11, 2019 at 05:27:43PM -0700, James Bottomley wrote:
> > > On Mon, 2019-03-11 at 16:54 -0700, Calvin Owens wrote:
> > > > e're having lots of problems with TPM commands timing out, and
> > > > we're seeing these problems across lots of different hardware
> > > > (both v1/v2).
> > > > 
> > > > I instrumented the driver to collect latency data, but I wasn't
> > > > able to find any specific timeout to fix: it seems like many of
> > > > them are too aggressive. So I tried replacing all the timeout
> > > > logic with a single universal long timeout, and found that makes
> > > > our TPMs 100% reliable.
> > > > 
> > > > Given that this timeout logic is very complex, problematic, and
> > > > appears to serve no real purpose, I propose simply deleting all
> > > > of it.
> > > 
> > > "no real purpose" is a bit strong given that all these timeouts are
> > > standards mandated.  The purpose stated by the standards is that
> > > there needs to be a way of differentiating the TPM crashed from the
> > > TPM is taking a very long time to respond.  For a normally
> > > functioning TPM it looks complex and unnecessary, but for a
> > > malfunctioning one it's a lifesaver.
> > 
> > Standards should be only followed when they make practical sense and
> > ignored when not. The range is only up to 2s anyway.
> 
> I don't disagree ... and I'm certainly not going to defend the TCG
> because I do think the complexity of some of its standards contributed
> to the lack of use of TPM 1.2.
> 
> However, I am saying we should root cause this problem rather than take
> a blind shot at the apparent timeout complexity.  My timeout
> instability is definitely related to the polling adjustments, so it's
> not unreasonable to think Facebooks might be as well.

Yeah, referring to my review comment, I think the very first thing
that should be done is to split patch into two. Then we can probably
give better feedback.

/Jarkko