Re: [Xen-devel] [PATCH XTF v4] Implement pv_read_some

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Mon, Jul 24, 2017 at 08:24:15AM +0200, Felix Schmoll wrote:
> Implement reading from PV console. Making use of polling.
> 
> Signed-off-by: Felix Schmoll <eggi.innovations@xxxxxxxxx>
> 
> ---
> Changed since v3:
>  * Add to comment: read function blocks if no data available
> ---
>  common/console.c      | 26 ++++++++++++++++++++++++++
>  include/xtf/console.h |  2 ++
>  2 files changed, 28 insertions(+)
> 
> diff --git a/common/console.c b/common/console.c
> index 7cb2361..b1a35f4 100644
> --- a/common/console.c
> +++ b/common/console.c
> @@ -46,6 +46,32 @@ static size_t pv_console_write_some(const char *buf, 
> size_t len)
>  }
>  
>  /*
> + * Read out data from the pv ring, either until buffer is filled or no
> + * more data are available. Might result in partial strings, depending
> + * on how xenconsoled passes in data.
> + *
> + * Will block if no data are available.
> + */
> +size_t pv_console_read_some(char *buf, size_t len)
> +{
> +    size_t s = 0;
> +    uint32_t cons, prod;
> +
> +    while ( !test_and_clear_bit(pv_evtchn, shared_info.evtchn_pending) ||
> +            (pv_ring->in_cons == pv_ring->in_prod) )
> +        hypercall_poll(pv_evtchn);
> +
> +    cons = pv_ring->in_cons, prod = LOAD_ACQUIRE(&pv_ring->in_prod);

Andrew pointed out on IRC this will degrade to

    cons = prod = LOAD_ACQUIRE(...);

I failed to spot this earlier.

See

http://en.cppreference.com/w/c/language/operator_precedence

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