Hi,
most people would use higher level tools lik libvirt [1] to easen the rough 
edges here.


I happened to use it the way you tried in the past, but I've also seen the 
error you mention.
The details depend on the used tpm, firmware and qemu versions.

For your use case you most likely will get away with adding a (even a
fake) cancel path:

$ touch /tmp/foo-cancel
$ qemu... -tpmdev 
passthrough,id=tpm0,path=/dev/tpm0,cancel-path=/tmp/foo-cancel -device 
tpm-tis,tpmdev=tpm0

It even is optional, but if it can't find it you will at least get the  warning.
Here from the man page:
       -tpmdev passthrough,id=id,path=path,cancel-path=cancel-path
              (Linux-host only) Enable access to the host's TPM using the 
passthrough driver.

              path specifies the path to the host's TPM device, i.e., on
a Linux host this would be /dev/tpm0. path is optional and by default
/dev/tpm0 is used.

              cancel-path  specifies  the path to the host TPM device's sysfs 
entry allowing for cancellation of an ongoing TPM command.  cancel-path is 
optional and by default QEMU will
              search for the sysfs entry to use.


Did this help or do you still think there is an issue in the package we'd need 
to fix?

[1]: https://libvirt.org/formatdomain.html#tpm-device

** Changed in: qemu (Ubuntu)
       Status: New => Incomplete

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https://bugs.launchpad.net/bugs/1944701

Title:
  tpm_passthrough is not working

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