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

[Jarkko Sakkinen <jarkko@kernel.org>]

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
> > > 
> > >  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?

For both, to give detailed review, good enough is their main branch
(i.e. as long as upstream accepts them I can use them).

> 
> > 
> > BR, Jarkko
> > 
> 
> Regards,
> Arun Menon
> 

BR, Jarkko