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

From: Mimi Zohar <zohar@linux.ibm.com>
To: "James Bottomley" <James.Bottomley@HansenPartnership.com>,
	"Jarkko Sakkinen" <jarkko.sakkinen@linux.intel.com>,
	"Peter Hüwe" <PeterHuewe@gmx.de>
Cc: Calvin Owens <calvinowens@fb.com>,
	Peter Huewe <peterhuewe@gmx.de>, Jason Gunthorpe <jgg@ziepe.ca>,
	Arnd Bergmann <arnd@arndb.de>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	linux-integrity@vger.kernel.org, linux-kernel@vger.kernel.org,
	kernel-team@fb.com
Subject: Re: [PATCH] tpm: Make timeout logic simpler and more robust
Date: Tue, 12 Mar 2019 12:59:29 -0400	[thread overview]
Message-ID: <1552409969.24794.68.camel@linux.ibm.com> (raw)
In-Reply-To: <1552401766.3083.3.camel@HansenPartnership.com>

On Tue, 2019-03-12 at 07:42 -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.

James, I thought Peter sent you a tis "debug" tool to help you debug
the problem you're seeing.  Whatever happened?

Mimi