Re: [RFC 0/4] tpm_crb: Add command and response buffer chunking support

[Jarkko Sakkinen <jarkko@kernel.org>]

On Fri, Apr 24, 2026 at 04:54:03PM -0400, Stefan Berger wrote:
> 
> 
> On 4/23/26 6:43 AM, Arun Menon wrote:
> > On Wed, Apr 08, 2026 at 11:34:54AM +0300, Jarkko Sakkinen wrote:
> > > On Tue, Mar 24, 2026 at 06:11:11PM +0530, Arun Menon wrote:
> > > > Hi Jarkko,
> > > > 
> > > > On Tue, Mar 24, 2026 at 12:41:26PM +0200, Jarkko Sakkinen wrote:
> > > > > On Tue, Mar 24, 2026 at 12:47:59PM +0530, Arun Menon wrote:
> > > > > > The new version of TCG TPM v185 (currently under review [1]) supports
> > > > > > sending data/commands in chunks for the CRB (Command Response Buffer)
> > > > > > interface. This is in line with the initiative to support PQC algorithms.
> > > > > > 
> > > > > > This series implements the logic to send and receive larger TPM
> > > > > > cmd/rsp between the linux guest and the TPM backend in chunks.
> > > > > > Currently, the TPM CRB driver is limited by the physical size of the
> > > > > > MMIO window. When userspace attempts to send a payload that exceeds this
> > > > > > size, the driver rejects it.
> > > > > > 
> > > > > > This series introduces chunking support. The driver now checks the CRB
> > > > > > interface capability for CRB_INTF_CAP_CRB_CHUNK. If supported by the
> > > > > > backend, the driver will slice oversized commands into MMIO-sized
> > > > > > chunks, signalling the backend via CRB_START_NEXT_CHUNK, and finalizing
> > > > > > with CRB_START_INVOKE. Responses are also read back in a similar chunked
> > > > > > manner.
> > > > > > 
> > > > > > If the backend does not support chunking, the driver retains its legacy
> > > > > > behaviour and enforces the standard size limits.
> > > > > > 
> > > > > > This feature also requires the QEMU to interpret the data in chunks and
> > > > > > forward it to the TPM backend and subsequently dispatch the TPM response
> > > > > > in chunks back to the linux guest. This is implemented in [2]
> > > > > > 
> > > > > > [1] https://trustedcomputinggroup.org/wp-content/uploads/PC-Client-Specific-Platform-TPM-Profile-for-TPM-2p0-v1p07_rc1_121225.pdf
> > > > > > [2] https://lore.kernel.org/qemu-devel/20260319135316.37412-1-armenon@redhat.com/
> > > > > > 
> > > > > > Arun Menon (4):
> > > > > >    tpm_crb: Add definition of TPM CRB chunking fields
> > > > > >    tpm_crb: Add new wrapper function to invoke start method
> > > > > >    tpm_crb: Implement command and response chunking logic
> > > > > >    tpm: Increase TPM_BUFSIZE to 64kB for chunking support
> 
> 64kb? I am only increasing the TPM buffer to 8kb. More does not seem to be
> necessary.
> 
> > > > > > 
> > > > > >   drivers/char/tpm/tpm.h     |   2 +-
> > > > > >   drivers/char/tpm/tpm_crb.c | 194 ++++++++++++++++++++++++++-----------
> > > > > >   2 files changed, 137 insertions(+), 59 deletions(-)
> > > > > > 
> > > > > > -- 
> > > > > > 2.53.0
> > > > > > 
> > > > > 
> > > > > When QEMU has the feature available?
> > > > 
> > > > The QEMU patches are in review at the moment,
> > > > here is the link: https://lore.kernel.org/qemu-devel/20260319135316.37412-1-armenon@redhat.com/
> > > > Hoping to have them merged soon.
> > > 
> > > Right, and additional question: what about swtpm?
> 
> I am waiting for https://github.com/trustedComputingGroup/tpm to show rev185
> with PQC support so that I can merge my patches based on 'their' PQC support
> into the public libtpms repo.
> 

Oh, great to hear, thanks for heads up.

Whenever that happens I'll update BuildRoot packages for libtpms and swtpm.

BR, Jarkko