[PATCH v8 2/3] tty: hvc: pass DMA capable memory to put_chars()

[Daniel Axtens]

Xianting Tian <xianting.tian at linux.alibaba.com> writes:

> As well known, hvc backend driver(eg, virtio-console) can register its
> operations to hvc framework. The operations can contain put_chars(),
> get_chars() and so on.
>
> Some hvc backend may do dma in its operations. eg, put_chars() of
> virtio-console. But in the code of hvc framework, it may pass DMA
> incapable memory to put_chars() under a specific configuration, which
> is explained in commit c4baad5029(virtio-console: avoid DMA from stack):

We could also run into issues on powerpc where Andrew is working on
adding vmap-stack but the opal hvc driver assumes that it is passed a
buffer which is not in vmalloc space but in the linear mapping. So it
would be good to fix this (or more clearly document what drivers can
expect).

> 1, c[] is on stack,
>    hvc_console_print():
> 	char c[N_OUTBUF] __ALIGNED__;
> 	cons_ops[index]->put_chars(vtermnos[index], c, i);
> 2, ch is on stack,
>    static void hvc_poll_put_char(,,char ch)
>    {
> 	struct tty_struct *tty = driver->ttys[0];
> 	struct hvc_struct *hp = tty->driver_data;
> 	int n;
>
> 	do {
> 		n = hp->ops->put_chars(hp->vtermno, &ch, 1);
> 	} while (n <= 0);
>    }
>
> Commit c4baad5029 is just the fix to avoid DMA from stack memory, which
> is passed to virtio-console by hvc framework in above code. But I think
> the fix is aggressive, it directly uses kmemdup() to alloc new buffer
> from kmalloc area and do memcpy no matter the memory is in kmalloc area
> or not. But most importantly, it should better be fixed in the hvc
> framework, by changing it to never pass stack memory to the put_chars()
> function in the first place. Otherwise, we still face the same issue if
> a new hvc backend using dma added in the future.
>
> In this patch, we make 'char out_buf[N_OUTBUF]' and 'chat out_ch' part
> of 'struct hvc_struct', so both two buf are no longer the stack memory.
> we can use it in above two cases separately.
>
> Introduce another array(cons_outbufs[]) for buffer pointers next to
> the cons_ops[] and vtermnos[] arrays. With the array, we can easily find
> the buffer, instead of traversing hp list.
>
> With the patch, we can remove the fix c4baad5029.
>
> Signed-off-by: Xianting Tian <xianting.tian at linux.alibaba.com>
> Reviewed-by: Shile Zhang <shile.zhang at linux.alibaba.com>

>  struct hvc_struct {
>  	struct tty_port port;
>  	spinlock_t lock;
>  	int index;
>  	int do_wakeup;
> -	char *outbuf;
> -	int outbuf_size;
>  	int n_outbuf;
>  	uint32_t vtermno;
>  	const struct hv_ops *ops;
> @@ -48,6 +56,10 @@ struct hvc_struct {
>  	struct work_struct tty_resize;
>  	struct list_head next;
>  	unsigned long flags;
> +	char out_ch;
> +	char out_buf[N_OUTBUF] __ALIGNED__;
> +	int outbuf_size;
> +	char outbuf[0] __ALIGNED__;

I'm trying to understand this patch but I am finding it very difficult
to understand what the difference between `out_buf` and `outbuf`
(without the underscore) is supposed to be. `out_buf` is statically
sized and the size of `outbuf` is supposed to depend on the arguments to
hvc_alloc(), but I can't quite figure out what the roles of each one are
and their names are confusingly similiar!

I looked briefly at the older revisions of the series but it didn't make
things much clearer.

Could you give them clearer names?

Also, looking at Documentation/process/deprecated.rst, it looks like
maybe we want to use a 'flexible array member' instead:

.. note:: If you are using struct_size() on a structure containing a zero-length
        or a one-element array as a trailing array member, please refactor such
        array usage and switch to a `flexible array member
        <#zero-length-and-one-element-arrays>`_ instead.

I think we want:

> +	char outbuf[] __ALIGNED__;

Kind regards,
Daniel