Re: [Qemu-devel] [PATCH V4 02/10] Add TPM (frontend) hardware interface (TPM TIS) to Qemu

["Serge E. Hallyn" <serge@hallyn.com>]

Quoting Stefan Berger (stefanb@linux.vnet.ibm.com):
> This patch adds the main code of the TPM frontend driver, the TPM TIS
> interface, to Qemu. The code is largely based on my previous implementation
> for Xen but has been significantly extended to meet the standard's
> requirements, such as the support for changing of localities and all the
> functionality of the available flags.
> 
> Communication with the backend (i.e., for Xen or the libtpms-based one)
> is cleanly separated through an interface which the backend driver needs
> to implement.
> 
> The TPM TIS driver's backend was previously chosen in the code added
> to arch_init. The frontend holds a pointer to the chosen backend (interface).
> 
> Communication with the backend is largely based on signals and conditions.
> Whenever the frontend has collected a complete packet, it will signal
> the backend, which then starts processing the command. Once the result
> has been returned, the backend invokes a callback function
> (tis_tpm_receive_cb()).
> 
> The one tricky part is support for VM suspend while the TPM is processing
> a command. In this case the frontend driver is waiting for the backend
> to return the result of the last command before shutting down. It waits
> on a condition for a signal from the backend, which is delivered in 
> tis_tpm_receive_cb().
> 
> Testing the proper functioning of the different flags and localities 
> cannot be done from user space when running in Linux for example, since
> access to the address space of the TPM TIS interface is not possible. Also
> the Linux driver itself does not exercise all functionality. So, for
> testing there is a fairly extensive test suite as part of the SeaBIOS patches
> since from within the BIOS one can have full access to all the TPM's registers.
> 
> v3:
>   - prefixing functions with tis_
>   - added a function to the backend interface 'early_startup_tpm' that
>     allows to detect the presence of the block storage and gracefully fails
>     Qemu if it's not available. This works with migration using shared
>     storage but doesn't support migration with block storage migration.
>     For encyrypted QCoW2 and in case of a snapshot resue the late_startup_tpm
>     interface function is called
> 
> Signed-off-by: Stefan Berger <stefanb@linux.vnet.ibm.com>

Most of this is pretty foreign to me so this doesn't mean much, but

Acked-by: Serge Hallyn <serge.hallyn@ubuntu.com>

> +/*

Worth pointing out here that this is called with mutex held.

> + * read a byte of response data
> + */
> +static uint32_t tis_data_read(TPMState *s, uint8_t locty)
> +{
> +    uint32_t ret = TPM_NO_DATA_BYTE;
> +    uint16_t len;
> +
> +    if ((s->loc[locty].sts & STS_DATA_AVAILABLE)) {
> +        len = tis_get_size_from_buffer(&s->loc[locty].r_buffer);
> +
> +        ret = s->loc[locty].r_buffer.buffer[s->loc[locty].r_offset++];
> +        if (s->loc[locty].r_offset >= len) {
> +            /* got last byte */
> +            s->loc[locty].sts = STS_VALID;