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Subject: Re: [PATCH v11 2/3] tty: hvc: pass DMA capable memory to put_chars()
To:     gregkh@linuxfoundation.org, jirislaby@kernel.org, amit@kernel.org,
        arnd@arndb.de, osandov@fb.com
Cc:     shile.zhang@linux.alibaba.com, linuxppc-dev@lists.ozlabs.org,
        virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org
References: <20211015024658.1353987-1-xianting.tian@linux.alibaba.com>
 <20211015024658.1353987-3-xianting.tian@linux.alibaba.com>
From:   Xianting Tian <xianting.tian@linux.alibaba.com>
Message-ID: <d56c2c23-3e99-417b-8144-cf1bb31b5f6d@linux.alibaba.com>
Date:   Wed, 20 Oct 2021 16:47:23 +0800
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hi Greg，

Could I get  your comments of this new version patches？ thanks

在 2021/10/15 上午10:46, Xianting Tian 写道:
> As well known, hvc backend can register its opertions to hvc backend.
> the operations 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):
> 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 furture.
>
> In this patch, add 'char cons_outbuf[]' as part of 'struct hvc_struct',
> so hp->cons_outbuf is no longer the stack memory, we can use it in above
> cases safely. We also add lock to protect cons_outbuf instead of using
> the global lock of hvc.
>
> Introduce another array(cons_hvcs[]) for hvc pointers next to the
> cons_ops[] and vtermnos[] arrays. With the array, we can easily find
> hvc's cons_outbuf and its lock.
>
> With the patch, we can revert the fix c4baad5029.
>
> Signed-off-by: Xianting Tian <xianting.tian@linux.alibaba.com>
> Signed-off-by: Shile Zhang <shile.zhang@linux.alibaba.com>
> ---
>   drivers/tty/hvc/hvc_console.c | 36 ++++++++++++++++++++---------------
>   drivers/tty/hvc/hvc_console.h | 21 +++++++++++++++++++-
>   2 files changed, 41 insertions(+), 16 deletions(-)
>
> diff --git a/drivers/tty/hvc/hvc_console.c b/drivers/tty/hvc/hvc_console.c
> index 5957ab728..11f2463a1 100644
> --- a/drivers/tty/hvc/hvc_console.c
> +++ b/drivers/tty/hvc/hvc_console.c
> @@ -41,16 +41,6 @@
>    */
>   #define HVC_CLOSE_WAIT (HZ/100) /* 1/10 of a second */
>   
> -/*
> - * These sizes are most efficient for vio, because they are the
> - * native transfer size. We could make them selectable in the
> - * future to better deal with backends that want other buffer sizes.
> - */
> -#define N_OUTBUF	16
> -#define N_INBUF		16
> -
> -#define __ALIGNED__ __attribute__((__aligned__(L1_CACHE_BYTES)))
> -
>   static struct tty_driver *hvc_driver;
>   static struct task_struct *hvc_task;
>   
> @@ -142,6 +132,7 @@ static int hvc_flush(struct hvc_struct *hp)
>   static const struct hv_ops *cons_ops[MAX_NR_HVC_CONSOLES];
>   static uint32_t vtermnos[MAX_NR_HVC_CONSOLES] =
>   	{[0 ... MAX_NR_HVC_CONSOLES - 1] = -1};
> +static struct hvc_struct *cons_hvcs[MAX_NR_HVC_CONSOLES];
>   
>   /*
>    * Console APIs, NOT TTY.  These APIs are available immediately when
> @@ -151,9 +142,11 @@ static uint32_t vtermnos[MAX_NR_HVC_CONSOLES] =
>   static void hvc_console_print(struct console *co, const char *b,
>   			      unsigned count)
>   {
> -	char c[N_OUTBUF] __ALIGNED__;
> +	char *c;
>   	unsigned i = 0, n = 0;
>   	int r, donecr = 0, index = co->index;
> +	unsigned long flags;
> +	struct hvc_struct *hp;
>   
>   	/* Console access attempt outside of acceptable console range. */
>   	if (index >= MAX_NR_HVC_CONSOLES)
> @@ -163,6 +156,13 @@ static void hvc_console_print(struct console *co, const char *b,
>   	if (vtermnos[index] == -1)
>   		return;
>   
> +	hp = cons_hvcs[index];
> +	if (!hp)
> +		return;
> +
> +	c = hp->cons_outbuf;
> +
> +	spin_lock_irqsave(&hp->cons_outbuf_lock, flags);
>   	while (count > 0 || i > 0) {
>   		if (count > 0 && i < sizeof(c)) {
>   			if (b[n] == '\n' && !donecr) {
> @@ -191,6 +191,7 @@ static void hvc_console_print(struct console *co, const char *b,
>   			}
>   		}
>   	}
> +	spin_unlock_irqrestore(&hp->cons_outbuf_lock, flags);
>   	hvc_console_flush(cons_ops[index], vtermnos[index]);
>   }
>   
> @@ -878,9 +879,13 @@ static void hvc_poll_put_char(struct tty_driver *driver, int line, char ch)
>   	struct tty_struct *tty = driver->ttys[0];
>   	struct hvc_struct *hp = tty->driver_data;
>   	int n;
> +	unsigned long flags;
>   
>   	do {
> -		n = hp->ops->put_chars(hp->vtermno, &ch, 1);
> +		spin_lock_irqsave(&hp->cons_outbuf_lock, flags);
> +		hp->cons_outbuf[0] = ch;
> +		n = hp->ops->put_chars(hp->vtermno, &hp->cons_outbuf[0], 1);
> +		spin_unlock_irqrestore(&hp->cons_outbuf_lock, flags);
>   	} while (n <= 0);
>   }
>   #endif
> @@ -922,8 +927,7 @@ struct hvc_struct *hvc_alloc(uint32_t vtermno, int data,
>   			return ERR_PTR(err);
>   	}
>   
> -	hp = kzalloc(ALIGN(sizeof(*hp), sizeof(long)) + outbuf_size,
> -			GFP_KERNEL);
> +	hp = kzalloc(struct_size(hp, outbuf, outbuf_size), GFP_KERNEL);
>   	if (!hp)
>   		return ERR_PTR(-ENOMEM);
>   
> @@ -931,13 +935,13 @@ struct hvc_struct *hvc_alloc(uint32_t vtermno, int data,
>   	hp->data = data;
>   	hp->ops = ops;
>   	hp->outbuf_size = outbuf_size;
> -	hp->outbuf = &((char *)hp)[ALIGN(sizeof(*hp), sizeof(long))];
>   
>   	tty_port_init(&hp->port);
>   	hp->port.ops = &hvc_port_ops;
>   
>   	INIT_WORK(&hp->tty_resize, hvc_set_winsz);
>   	spin_lock_init(&hp->lock);
> +	spin_lock_init(&hp->cons_outbuf_lock);
>   	mutex_lock(&hvc_structs_mutex);
>   
>   	/*
> @@ -964,6 +968,7 @@ struct hvc_struct *hvc_alloc(uint32_t vtermno, int data,
>   	if (i < MAX_NR_HVC_CONSOLES) {
>   		cons_ops[i] = ops;
>   		vtermnos[i] = vtermno;
> +		cons_hvcs[i] = hp;
>   	}
>   
>   	list_add_tail(&(hp->next), &hvc_structs);
> @@ -988,6 +993,7 @@ int hvc_remove(struct hvc_struct *hp)
>   	if (hp->index < MAX_NR_HVC_CONSOLES) {
>   		vtermnos[hp->index] = -1;
>   		cons_ops[hp->index] = NULL;
> +		cons_hvcs[hp->index] = NULL;
>   	}
>   
>   	/* Don't whack hp->irq because tty_hangup() will need to free the irq. */
> diff --git a/drivers/tty/hvc/hvc_console.h b/drivers/tty/hvc/hvc_console.h
> index 18d005814..2c32ab67b 100644
> --- a/drivers/tty/hvc/hvc_console.h
> +++ b/drivers/tty/hvc/hvc_console.h
> @@ -32,12 +32,21 @@
>    */
>   #define HVC_ALLOC_TTY_ADAPTERS	8
>   
> +/*
> + * These sizes are most efficient for vio, because they are the
> + * native transfer size. We could make them selectable in the
> + * future to better deal with backends that want other buffer sizes.
> + */
> +#define N_OUTBUF	16
> +#define N_INBUF		16
> +
> +#define __ALIGNED__ __attribute__((__aligned__(L1_CACHE_BYTES)))
> +
>   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;
> @@ -48,6 +57,16 @@ struct hvc_struct {
>   	struct work_struct tty_resize;
>   	struct list_head next;
>   	unsigned long flags;
> +
> +	/*
> +	 * the buf and its lock are used in hvc console api for putting chars,
> +	 * and also used in hvc_poll_put_char() for putting single char.
> +	 */
> +	spinlock_t cons_outbuf_lock;
> +	char cons_outbuf[N_OUTBUF] __ALIGNED__;
> +
> +	/* the buf is used for putting chars to tty */
> +	char outbuf[] __ALIGNED__;
>   };
>   
>   /* implemented by a low level driver */