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Date: Tue, 21 Feb 2012 00:02:01 +0200
From: "Michael S. Tsirkin" <mst@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH V14 2/7] Add TPM (frontend) hardware
 interface (TPM TIS) to Qemu
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To: Stefan Berger <stefanb@linux.vnet.ibm.com>
Cc: qemu-devel@nongnu.org, andreas.niederl@iaik.tugraz.at

On Wed, Dec 14, 2011 at 08:43:17AM -0500, Stefan Berger wrote:
> +/*
> + * Send a TPM request.
> + * Call this with the state_lock held so we can sync with the receive
> + * callback.
> + */
> +static void tpm_tis_tpm_send(TPMState *s, uint8_t locty)
> +{
> +    TPMTISState *tis = &s->s.tis;
> +
> +    tpm_tis_show_buffer(&tis->loc[locty].w_buffer, "tpm_tis: To TPM");
> +
> +    s->command_locty = locty;
> +    s->cmd_locty     = &tis->loc[locty];
> +
> +    /* w_offset serves as length indicator for length of data;
> +       it's reset when the response comes back */
> +    tis->loc[locty].status = TPM_TIS_STATUS_EXECUTION;
> +    tis->loc[locty].sts &= ~TPM_TIS_STS_EXPECT;
> +
> +    s->to_tpm_execute = true;
> +    qemu_cond_signal(&s->to_tpm_cond);
> +}

What happens IIUC is that frondend sets to_tpm_execute
and signals a condition, and backend clears it
and waits on a condition.

So how about moving all the signalling
and locking out to backend, and have frontend
invoke a callback to signal it?

The whole threading thing then becomes a work-around
for a backend that does not support select,
instead of spilling out into frontend?

-- 
MST