LKML Archive on
help / color / mirror / Atom feed
From: Xianting TIan <>
To: Greg KH <>
Subject: Re: [PATCH v6 1/2] tty: hvc: pass DMA capable memory to put_chars()
Date: Fri, 13 Aug 2021 19:27:09 +0800	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <YRYeyeZ/>

在 2021/8/13 下午3:27, Greg KH 写道:
> On Thu, Aug 12, 2021 at 05:45:31PM +0800, 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.
>> 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_outbuf[]) for the hp->c pointers next to
>> the cons_ops[] and vtermnos[] arrays.
>> With the patch, we can remove the fix c4baad5029.
>> Signed-off-by: Xianting Tian <>
>> Tested-by: Xianting Tian <>
> As the build shows, you obviously did not test this code :(
> Also, no need to add a tested-by line as that should be implicit if you
> wrote and signed off on it.
> I am going to ask you to get some help from some other developers at
> your company, and get them to test and sign off on this series before
> sending it out again, as there seems to be a bit of a disconnect as to
> what is actually needed to do when sending a patch for us to review.
> That is now a requirement for us to be able to take your changes here.
> thanks,

Sorry for this.

I tested V1-V4,  But for V6, I take it for granted that there is no 
problem when I just switch to use array(cons_outbuf[]).  I indeed didn't 
test it:(

I will test it and find virtualization test expert to test again before 
sending next patch.

> greg k-h

  reply	other threads:[~2021-08-13 11:27 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-12  9:45 [PATCH v6 0/2] make hvc pass dma capable memory to its backend Xianting Tian
2021-08-12  9:45 ` [PATCH v6 1/2] tty: hvc: pass DMA capable memory to put_chars() Xianting Tian
2021-08-12 12:26   ` kernel test robot
2021-08-13  5:53     ` Jiri Slaby
2021-08-13  6:04       ` Xianting TIan
2021-08-13 16:07       ` Xianting TIan
2021-08-13  7:27   ` Greg KH
2021-08-13 11:27     ` Xianting TIan [this message]
2021-08-12  9:45 ` [PATCH v6 2/2] virtio-console: remove unnecessary kmemdup() Xianting Tian

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \ \ \ \ \ \ \ \ \ \ \
    --subject='Re: [PATCH v6 1/2] tty: hvc: pass DMA capable memory to put_chars()' \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).