[PATCH v7 0/2] make hvc pass dma capable memory to its backend

[PATCH v7 0/2] make hvc pass dma capable memory to its backend

[Xianting Tian]

Dear all,

This patch series make hvc framework pass DMA capable memory to
put_chars() of hvc backend(eg, virtio-console), and revert commit
c4baad5029 ("virtio-console: avoid DMA from stack”)

V1
virtio-console: avoid DMA from vmalloc area
https://lkml.org/lkml/2021/7/27/494

For v1 patch, Arnd Bergmann suggests to fix the issue in the first
place:
Make hvc pass DMA capable memory to put_chars()
The fix suggestion is included in v2.

V2
[PATCH 1/2] tty: hvc: pass DMA capable memory to put_chars()
https://lkml.org/lkml/2021/8/1/8
[PATCH 2/2] virtio-console: remove unnecessary kmemdup()
https://lkml.org/lkml/2021/8/1/9

For v2 patch, Arnd Bergmann suggests to make new buf part of the
hvc_struct structure, and fix the compile issue.
The fix suggestion is included in v3.

V3
[PATCH v3 1/2] tty: hvc: pass DMA capable memory to put_chars()
https://lkml.org/lkml/2021/8/3/1347
[PATCH v3 2/2] virtio-console: remove unnecessary kmemdup()
https://lkml.org/lkml/2021/8/3/1348

For v3 patch, Jiri Slaby suggests to make 'char c[N_OUTBUF]' part of
hvc_struct, and make 'hp->outbuf' aligned and use struct_size() to
calculate the size of hvc_struct. The fix suggestion is included in
v4.

V4
[PATCH v4 0/2] make hvc pass dma capable memory to its backend
https://lkml.org/lkml/2021/8/5/1350
[PATCH v4 1/2] tty: hvc: pass DMA capable memory to put_chars()
https://lkml.org/lkml/2021/8/5/1351
[PATCH v4 2/2] virtio-console: remove unnecessary kmemdup()
https://lkml.org/lkml/2021/8/5/1352

For v4 patch, Arnd Bergmann suggests to introduce another
array(cons_outbuf[]) for the buffer pointers next to the cons_ops[]
and vtermnos[] arrays. This fix included in this v5 patch.

V5
Arnd Bergmann suggests to use "L1_CACHE_BYTES" as dma alignment,
use 'sizeof(long)' as dma alignment is wrong. fix it in v6.

V6
It contains coding error, fix it in v7 and it worked normally
according to test result.

drivers/tty/hvc/hvc_console.c | 39 +++++++++++++++++++++--------------
drivers/tty/hvc/hvc_console.h | 16 ++++++++++++--
drivers/tty/hvc/hvc_console.h | 16 ++++++++++++--
3 file changed

[PATCH v7 1/2] tty: hvc: pass DMA capable memory to put_chars()

[Xianting Tian]

As well known, hvc backend can register its opertions to hvc backend.
the opertions contain put_chars(), get_chars() and so on.

Some hvc backend may do dma in its opertions. 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.

We make 'char c[N_OUTBUF]' part of 'struct hvc_struct', so hp->c is no
longer the stack memory. we can use it in above two cases.

Other fix is use L1_CACHE_BYTES as the alignment, use 'sizeof(long)' as
dma alignment is wrong. And use struct_size() to calculate size of
hvc_struct.

Introduce another array(cons_hvcs[]) for hvc_struct pointers next to
the cons_ops[] and vtermnos[] arrays. With the array, we can easily find
the hp, instead of traversing hp list.

With the patch, we can remove the fix c4baad5029.

We tested the patch, it worked normally.

Signed-off-by: Xianting Tian <xianting.tian@linux.alibaba.com>
---
 drivers/tty/hvc/hvc_console.c | 39 +++++++++++++++++++++--------------
 drivers/tty/hvc/hvc_console.h | 16 ++++++++++++--
 2 files changed, 38 insertions(+), 17 deletions(-)

diff --git a/drivers/tty/hvc/hvc_console.c b/drivers/tty/hvc/hvc_console.c
index 5bb8c4e44..b882ceb5f 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__(sizeof(long))))
-
 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 || !hp->c)
+		return;
+
+	c = hp->c;
+
+	spin_lock_irqsave(&hp->c_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->c_lock, flags);
 	hvc_console_flush(cons_ops[index], vtermnos[index]);
 }
 
@@ -878,9 +879,16 @@ 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;
+
+	if (!hp || !hp->c)
+		return;
 
 	do {
-		n = hp->ops->put_chars(hp->vtermno, &ch, 1);
+		spin_lock_irqsave(&hp->c_lock, flags);
+		hp->c[0] = ch;
+		n = hp->ops->put_chars(hp->vtermno, hp->c, 1);
+		spin_unlock_irqrestore(&hp->c_lock, flags);
 	} while (n <= 0);
 }
 #endif
@@ -922,8 +930,7 @@ 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 +938,13 @@ 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->c_lock);
 	mutex_lock(&hvc_structs_mutex);
 
 	/*
@@ -964,6 +971,7 @@ 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 +996,7 @@ 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..97a5f1e0f 100644
--- a/drivers/tty/hvc/hvc_console.h
+++ b/drivers/tty/hvc/hvc_console.h
@@ -32,13 +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;
 	const struct hv_ops *ops;
@@ -48,6 +56,10 @@ struct hvc_struct {
 	struct work_struct tty_resize;
 	struct list_head next;
 	unsigned long flags;
+	spinlock_t c_lock;
+	char c[N_OUTBUF] __ALIGNED__;
+	int outbuf_size;
+	char outbuf[0] __ALIGNED__;
 };
 
 /* implemented by a low level driver */
-- 
2.17.1

[PATCH v7 2/2] virtio-console: remove unnecessary kmemdup()

[Xianting Tian]

hvc framework will never pass stack memory to the put_chars() function,
So the calling of kmemdup() is unnecessary, we can remove it.

This revert commit c4baad5029 ("virtio-console: avoid DMA from stack")

Signed-off-by: Xianting Tian <xianting.tian@linux.alibaba.com>
---
 drivers/char/virtio_console.c | 12 ++----------
 1 file changed, 2 insertions(+), 10 deletions(-)

diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
index 7eaf303a7..4ed3ffb1d 100644
--- a/drivers/char/virtio_console.c
+++ b/drivers/char/virtio_console.c
@@ -1117,8 +1117,6 @@ static int put_chars(u32 vtermno, const char *buf, int count)
 {
 	struct port *port;
 	struct scatterlist sg[1];
-	void *data;
-	int ret;
 
 	if (unlikely(early_put_chars))
 		return early_put_chars(vtermno, buf, count);
@@ -1127,14 +1125,8 @@ static int put_chars(u32 vtermno, const char *buf, int count)
 	if (!port)
 		return -EPIPE;
 
-	data = kmemdup(buf, count, GFP_ATOMIC);
-	if (!data)
-		return -ENOMEM;
-
-	sg_init_one(sg, data, count);
-	ret = __send_to_port(port, sg, 1, count, data, false);
-	kfree(data);
-	return ret;
+	sg_init_one(sg, buf, count);
+	return __send_to_port(port, sg, 1, count, (void *)buf, false);
 }
 
 /*
-- 
2.17.1

Re: [PATCH v7 1/2] tty: hvc: pass DMA capable memory to put_chars()

[Greg KH]

On Tue, Aug 17, 2021 at 09:22:59PM +0800, Xianting Tian wrote:
> We tested the patch, it worked normally.

Who is "we"?

> Signed-off-by: Xianting Tian <xianting.tian@linux.alibaba.com>

Like I said before, I need another developer from your company to review
and sign-off on this patch (and the other one), before I am willing to
look at it, based on the previous mistakes that have happened here.

thanks,

greg k-h

Re: [PATCH v7 1/2] tty: hvc: pass DMA capable memory to put_chars()

[kernel test robot]

[-- Attachment #1: Type: text/plain, Size: 4844 bytes --]

Hi Xianting,

Thank you for the patch! Perhaps something to improve:

[auto build test WARNING on tty/tty-testing]
[also build test WARNING on char-misc/char-misc-testing soc/for-next v5.14-rc6 next-20210817]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch]

url:    https://github.com/0day-ci/linux/commits/Xianting-Tian/make-hvc-pass-dma-capable-memory-to-its-backend/20210817-212556
base:   https://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty.git tty-testing
config: i386-randconfig-r021-20210816 (attached as .config)
compiler: clang version 14.0.0 (https://github.com/llvm/llvm-project 2c6448cdc2f68f8c28fd0bd9404182b81306e6e6)
reproduce (this is a W=1 build):
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # https://github.com/0day-ci/linux/commit/f12c3bee9f2413ed7643d858b40ce2337329fdae
        git remote add linux-review https://github.com/0day-ci/linux
        git fetch --no-tags linux-review Xianting-Tian/make-hvc-pass-dma-capable-memory-to-its-backend/20210817-212556
        git checkout f12c3bee9f2413ed7643d858b40ce2337329fdae
        # save the attached .config to linux build tree
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross ARCH=i386 

If you fix the issue, kindly add following tag as appropriate
Reported-by: kernel test robot <lkp@intel.com>

All warnings (new ones prefixed by >>):

   clang-14: warning: optimization flag '-falign-jumps=0' is not supported [-Wignored-optimization-argument]
   In file included from drivers/tty/hvc/hvc_console.c:15:
   In file included from include/linux/kbd_kern.h:5:
   In file included from include/linux/tty.h:5:
   In file included from include/linux/fs.h:6:
   In file included from include/linux/wait_bit.h:8:
   In file included from include/linux/wait.h:9:
   In file included from include/linux/spinlock.h:51:
   In file included from include/linux/preempt.h:78:
   In file included from arch/x86/include/asm/preempt.h:7:
   In file included from include/linux/thread_info.h:60:
   arch/x86/include/asm/thread_info.h:172:13: warning: calling '__builtin_frame_address' with a nonzero argument is unsafe [-Wframe-address]
           oldframe = __builtin_frame_address(1);
                      ^~~~~~~~~~~~~~~~~~~~~~~~~~
   arch/x86/include/asm/thread_info.h:174:11: warning: calling '__builtin_frame_address' with a nonzero argument is unsafe [-Wframe-address]
                   frame = __builtin_frame_address(2);
                           ^~~~~~~~~~~~~~~~~~~~~~~~~~
>> drivers/tty/hvc/hvc_console.c:160:18: warning: address of array 'hp->c' will always evaluate to 'true' [-Wpointer-bool-conversion]
           if (!hp || !hp->c)
                      ~~~~~^
   3 warnings generated.


vim +160 drivers/tty/hvc/hvc_console.c

   136	
   137	/*
   138	 * Console APIs, NOT TTY.  These APIs are available immediately when
   139	 * hvc_console_setup() finds adapters.
   140	 */
   141	
   142	static void hvc_console_print(struct console *co, const char *b,
   143				      unsigned count)
   144	{
   145		char *c;
   146		unsigned i = 0, n = 0;
   147		int r, donecr = 0, index = co->index;
   148		unsigned long flags;
   149		struct hvc_struct *hp;
   150	
   151		/* Console access attempt outside of acceptable console range. */
   152		if (index >= MAX_NR_HVC_CONSOLES)
   153			return;
   154	
   155		/* This console adapter was removed so it is not usable. */
   156		if (vtermnos[index] == -1)
   157			return;
   158	
   159		hp = cons_hvcs[index];
 > 160		if (!hp || !hp->c)
   161			return;
   162	
   163		c = hp->c;
   164	
   165		spin_lock_irqsave(&hp->c_lock, flags);
   166		while (count > 0 || i > 0) {
   167			if (count > 0 && i < sizeof(c)) {
   168				if (b[n] == '\n' && !donecr) {
   169					c[i++] = '\r';
   170					donecr = 1;
   171				} else {
   172					c[i++] = b[n++];
   173					donecr = 0;
   174					--count;
   175				}
   176			} else {
   177				r = cons_ops[index]->put_chars(vtermnos[index], c, i);
   178				if (r <= 0) {
   179					/* throw away characters on error
   180					 * but spin in case of -EAGAIN */
   181					if (r != -EAGAIN) {
   182						i = 0;
   183					} else {
   184						hvc_console_flush(cons_ops[index],
   185							      vtermnos[index]);
   186					}
   187				} else if (r > 0) {
   188					i -= r;
   189					if (i > 0)
   190						memmove(c, c+r, i);
   191				}
   192			}
   193		}
   194		spin_unlock_irqrestore(&hp->c_lock, flags);
   195		hvc_console_flush(cons_ops[index], vtermnos[index]);
   196	}
   197	

---
0-DAY CI Kernel Test Service, Intel Corporation
https://lists.01.org/hyperkitty/list/kbuild-all@lists.01.org

[-- Attachment #2: .config.gz --]
[-- Type: application/gzip, Size: 36668 bytes --]

Re: [PATCH v7 1/2] tty: hvc: pass DMA capable memory to put_chars()

[Xianting TIan]

在 2021/8/17 下午11:48, Greg KH 写道:
> On Tue, Aug 17, 2021 at 09:22:59PM +0800, Xianting Tian wrote:
>> We tested the patch, it worked normally.
> Who is "we"?
>
>> Signed-off-by: Xianting Tian <xianting.tian@linux.alibaba.com>
> Like I said before, I need another developer from your company to review
> and sign-off on this patch (and the other one), before I am willing to
> look at it, based on the previous mistakes that have happened here.
thanks, I will add the developer in v8 and also with fix a build 
warning, which I don't meet in my build process.
>
> thanks,
>
> greg k-h

Re: [PATCH v7 1/2] tty: hvc: pass DMA capable memory to put_chars()

[Jiri Slaby]

Hi,

On 17. 08. 21, 15:22, Xianting Tian wrote:
> As well known, hvc backend can register its opertions to hvc backend.
> the opertions contain put_chars(), get_chars() and so on.

"operations". And there too:

> Some hvc backend may do dma in its opertions. 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.
> 
> We make 'char c[N_OUTBUF]' part of 'struct hvc_struct', so hp->c is no
> longer the stack memory. we can use it in above two cases.

In fact, you need only a single char for the poll case 
(hvc_poll_put_char), so hvc_struct needs to contain only c, not an array.

OTOH, you need c[N_OUTBUF] in the console case (hvc_console_print), but 
not whole hvc_struct. So cons_hvcs should be an array of structs 
composed of only the lock and the buffer.

Hum.

Or maybe rethink and take care of the console case by kmemdup and be 
done with that case? What problem do you have with allocating 16 bytes? 
It should be quite easy and really fast (lockless) in most cases. On the 
contrary, your solution has to take a spinlock to access the global buffer.

> Other fix is use L1_CACHE_BYTES as the alignment, use 'sizeof(long)' as
> dma alignment is wrong. And use struct_size() to calculate size of
> hvc_struct.

This ought to be in separate patches.

thanks,
-- 
js
suse labs

Re: [PATCH v7 1/2] tty: hvc: pass DMA capable memory to put_chars()

[Xianting TIan]

在 2021/8/18 上午11:17, Jiri Slaby 写道:
> Hi,
>
> On 17. 08. 21, 15:22, Xianting Tian wrote:
>> As well known, hvc backend can register its opertions to hvc backend.
>> the opertions contain put_chars(), get_chars() and so on.
>
> "operations". And there too:
>
>> Some hvc backend may do dma in its opertions. 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.
>>
>> We make 'char c[N_OUTBUF]' part of 'struct hvc_struct', so hp->c is no
>> longer the stack memory. we can use it in above two cases.
>
> In fact, you need only a single char for the poll case 
> (hvc_poll_put_char), so hvc_struct needs to contain only c, not an array.
>
> OTOH, you need c[N_OUTBUF] in the console case (hvc_console_print), 
> but not whole hvc_struct. So cons_hvcs should be an array of structs 
> composed of only the lock and the buffer.
>
> Hum.
>
> Or maybe rethink and take care of the console case by kmemdup and be 
> done with that case? What problem do you have with allocating 16 
> bytes? It should be quite easy and really fast (lockless) in most 
> cases. On the contrary, your solution has to take a spinlock to access 
> the global buffer.

May be we can change hvc_struct as below,

struct hvc_struct {

         ...
         char out_ch;
         char c[N_OUTBUF] __ALIGNED__;
         int outbuf_size;
         char outbuf[0] __ALIGNED__;
};

c[N_OUTBUF]  is only used for hvc_console_print(); out_ch is only used 
for hvc_poll_put_char(). Thus no competition exits, the spinlock can be 
removed.

Then cons_hvcs array can only contains the buffer.

Is it OK for you?  thanks
>
>> Other fix is use L1_CACHE_BYTES as the alignment, use 'sizeof(long)' as
>> dma alignment is wrong. And use struct_size() to calculate size of
>> hvc_struct.
>
> This ought to be in separate patches.
OK, thanks
>
> thanks,

Re: [PATCH v7 1/2] tty: hvc: pass DMA capable memory to put_chars()

[Xianting TIan]

在 2021/8/18 上午11:17, Jiri Slaby 写道:
> Hi,
>
> On 17. 08. 21, 15:22, Xianting Tian wrote:
>> As well known, hvc backend can register its opertions to hvc backend.
>> the opertions contain put_chars(), get_chars() and so on.
>
> "operations". And there too:
>
>> Some hvc backend may do dma in its opertions. 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.
>>
>> We make 'char c[N_OUTBUF]' part of 'struct hvc_struct', so hp->c is no
>> longer the stack memory. we can use it in above two cases.
>
> In fact, you need only a single char for the poll case 
> (hvc_poll_put_char), so hvc_struct needs to contain only c, not an array.
>
> OTOH, you need c[N_OUTBUF] in the console case (hvc_console_print), 
> but not whole hvc_struct. So cons_hvcs should be an array of structs 
> composed of only the lock and the buffer.
>
> Hum.
>
> Or maybe rethink and take care of the console case by kmemdup and be 
> done with that case? What problem do you have with allocating 16 
> bytes? It should be quite easy and really fast (lockless) in most 
> cases. On the contrary, your solution has to take a spinlock to access 
> the global buffer.

As I replyed before,  this issue may can be solved just by adjust the 
alignment to L1_CACHE_BYTES or at least 16: 

#define __ALIGNED__ __attribute__((__aligned__(L1_CACHE_BYTES)))

Then, c[16] won't cross the pages, that is to say c[16]'s physical 
address is continuous.  Could you comment this?

I submitted v8,  I found it still can't solve ths issue, even we create 
'char out_buf[N_OUTBUF]' and 'chat out_ch' be part of 'struct 
hvc_struct', and use it separately, we still need lock to protect each 
buf. When we invloced lock, it will impact the hvc performance.

So we can back to the original intention of this solution, just fix the 
kmemdup issue in virtio_console driver?


>
>> Other fix is use L1_CACHE_BYTES as the alignment, use 'sizeof(long)' as
>> dma alignment is wrong. And use struct_size() to calculate size of
>> hvc_struct.
>
> This ought to be in separate patches.
>
> thanks,