Re: [PATCH v2] tpm: tpm2-space: Resize session and context buffers dynamically

From: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
To: Stefan Berger <stefanb@linux.ibm.com>
Cc: linux-integrity@vger.kernel.org,
	James Bottomley <James.Bottomley@HansenPartnership.com>,
	Peter Huewe <peterhuewe@gmx.de>, Jason Gunthorpe <jgg@ziepe.ca>,
	Arnd Bergmann <arnd@arndb.de>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Jerry Snitselaar <jsnitsel@redhat.com>,
	Sumit Garg <sumit.garg@linaro.org>,
	Alexey Klimov <aklimov@redhat.com>,
	open list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2] tpm: tpm2-space: Resize session and context buffers dynamically
Date: Thu, 2 Jul 2020 22:54:26 +0300	[thread overview]
Message-ID: <20200702195426.GA31291@linux.intel.com> (raw)
In-Reply-To: <5d786bca-6a0f-0c7f-e856-04dac2f280f6@linux.ibm.com>

On Fri, Jun 26, 2020 at 08:16:45AM -0400, Stefan Berger wrote:
> On 6/26/20 7:48 AM, Jarkko Sakkinen wrote:
> > On Thu, Jun 25, 2020 at 05:27:50PM -0400, Stefan Berger wrote:
> > > On 6/25/20 5:25 PM, Jarkko Sakkinen wrote:
> > > > On Thu, Jun 25, 2020 at 08:41:18AM -0400, Stefan Berger wrote:
> > > > > On 6/25/20 12:38 AM, Jarkko Sakkinen wrote:
> > > > > > Re-allocate context and session buffers when needed. Scale them in page
> > > > > > increments so that the reallocation is only seldomly required, and thus
> > > > > > causes minimal stress to the system. Add a static maximum limit of four
> > > > > > pages for buffer sizes.
> > > > > > 
> > > > > > Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
> > > > > > Suggested-by: Stefan Berger <stefanb@linux.ibm.com>
> > > > > > Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
> > > > > You don't want to try a fixes tag? None of the previous versions of this
> > > > > code will work with newer versions of the TPM 2 then...
> > > > It's not a regression.
> > > Ok, so distros will have to backport it.
> > Now that you mentioned PPC64 in some other email that would make this a
> > regression since x86 provides less space for keys than PPC64.
> > 
> > I studied PPC64 a bit and it actually allows max 256 kB page size, which
> > is too much for us, given that there is no accounting implemented for
> > TPM spaces (so far, should be done eventually).
> > 
> > So to summarize: 0 the idea would decrease the limit on PPC64 and
> > increase it on ther arch's.  `
> > 
> > Dynamic scaling is over to top for fixing the issue, which means that I
> > will just define static size of 16 kB for the buffer. We can reconsider
> > it if we hit the roof again.
> 
> 16kb is plenty of space for years to come. Maybe just enlarge the buffer for
> the regression and then do dynamic allocation as the final solution for the
> tip. I can try to test compile it on one or two long term stable kernels.
> Hopefully it applies cleanly. Simple test just in case you had a setup with
> a VM and libtpms master:

3*4096 bytes is based on absolutely nothing.

The chosen page size is based on the PPC64 default page size.

/Jarkko