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From: Xianting TIan <xianting.tian@linux.alibaba.com>
To: Jiri Slaby <jirislaby@kernel.org>,
	gregkh@linuxfoundation.org, amit@kernel.org, arnd@arndb.de,
	osandov@fb.com
Cc: clang-built-linux@googlegroups.com, kbuild-all@lists.01.org,
	linuxppc-dev@lists.ozlabs.org,
	virtualization@lists.linux-foundation.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v6 1/2] tty: hvc: pass DMA capable memory to put_chars()
Date: Sat, 14 Aug 2021 00:07:03 +0800	[thread overview]
Message-ID: <9f6b0b31-b2fd-0706-b40f-d7ffc5c48737@linux.alibaba.com> (raw)
In-Reply-To: <0c808001-7643-fdcf-66ba-738654ec0c21@kernel.org>


在 2021/8/13 下午1:53, Jiri Slaby 写道:
> Hi,
>
> On 12. 08. 21, 14:26, kernel test robot wrote:
>>>> drivers/tty/hvc/hvc_console.c:190:26: warning: variable 'hp' is 
>>>> uninitialized when used here [-Wuninitialized]
>>             spin_unlock_irqrestore(&hp->c_lock, flags);
>>                                     ^~
>>     drivers/tty/hvc/hvc_console.c:149:23: note: initialize the 
>> variable 'hp' to silence this warning
>>             struct hvc_struct *hp;
>>                                  ^
>>                                   = NULL
>
> So you clearly didn't test your change as it would crash quite 
> instantly. I wonder, where do you intend to get hp from in the 
> console::print() hook?
>
> thanks,
According to analysis, this issue may can be solved just by adjust the 
alignment to L1_CACHE_BYTES: 

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

Our analysis as below, the original __ALIGNED__ is sizeof(long) which is 
8 for 64bit cpu.
char c[N_OUTBUF] __ALIGNED__; //c[16] __ALIGNED__;

For 4K page, c[16] may cross the page when alignemnt is 8.

In the case the physical address of c[16] is noncontiguous.
|------8----|..........|-----8-----|    PAGE_1
..........................|-----------16----------| c[16]

.........................................|-----8-----|.............|-----8-----| 
PAGE_2
But when the alignment is L1_CACHE_BYTES(eg, 64), or at least 
N_OUTBUF(16), we have no dma issue as c[16] won't cross the page, the 
physical address of c[16] is contiguous.
|--------64--------|.........|--------64---------| PAGE_3

..................................|--c[16]--|

Could you help comments this?  thanks


WARNING: multiple messages have this Message-ID (diff)
From: Xianting TIan <xianting.tian@linux.alibaba.com>
To: Jiri Slaby <jirislaby@kernel.org>,
	gregkh@linuxfoundation.org, amit@kernel.org, arnd@arndb.de,
	osandov@fb.com
Cc: clang-built-linux@googlegroups.com, kbuild-all@lists.01.org,
	linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
	virtualization@lists.linux-foundation.org
Subject: Re: [PATCH v6 1/2] tty: hvc: pass DMA capable memory to put_chars()
Date: Sat, 14 Aug 2021 00:07:03 +0800	[thread overview]
Message-ID: <9f6b0b31-b2fd-0706-b40f-d7ffc5c48737@linux.alibaba.com> (raw)
In-Reply-To: <0c808001-7643-fdcf-66ba-738654ec0c21@kernel.org>


在 2021/8/13 下午1:53, Jiri Slaby 写道:
> Hi,
>
> On 12. 08. 21, 14:26, kernel test robot wrote:
>>>> drivers/tty/hvc/hvc_console.c:190:26: warning: variable 'hp' is 
>>>> uninitialized when used here [-Wuninitialized]
>>             spin_unlock_irqrestore(&hp->c_lock, flags);
>>                                     ^~
>>     drivers/tty/hvc/hvc_console.c:149:23: note: initialize the 
>> variable 'hp' to silence this warning
>>             struct hvc_struct *hp;
>>                                  ^
>>                                   = NULL
>
> So you clearly didn't test your change as it would crash quite 
> instantly. I wonder, where do you intend to get hp from in the 
> console::print() hook?
>
> thanks,
According to analysis, this issue may can be solved just by adjust the 
alignment to L1_CACHE_BYTES: 

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

Our analysis as below, the original __ALIGNED__ is sizeof(long) which is 
8 for 64bit cpu.
char c[N_OUTBUF] __ALIGNED__; //c[16] __ALIGNED__;

For 4K page, c[16] may cross the page when alignemnt is 8.

In the case the physical address of c[16] is noncontiguous.
|------8----|..........|-----8-----|    PAGE_1
..........................|-----------16----------| c[16]

.........................................|-----8-----|.............|-----8-----| 
PAGE_2
But when the alignment is L1_CACHE_BYTES(eg, 64), or at least 
N_OUTBUF(16), we have no dma issue as c[16] won't cross the page, the 
physical address of c[16] is contiguous.
|--------64--------|.........|--------64---------| PAGE_3

..................................|--c[16]--|

Could you help comments this?  thanks


WARNING: multiple messages have this Message-ID (diff)
From: Xianting TIan <xianting.tian@linux.alibaba.com>
To: kbuild-all@lists.01.org
Subject: Re: [PATCH v6 1/2] tty: hvc: pass DMA capable memory to put_chars()
Date: Sat, 14 Aug 2021 00:07:03 +0800	[thread overview]
Message-ID: <9f6b0b31-b2fd-0706-b40f-d7ffc5c48737@linux.alibaba.com> (raw)
In-Reply-To: <0c808001-7643-fdcf-66ba-738654ec0c21@kernel.org>

[-- Attachment #1: Type: text/plain, Size: 1847 bytes --]


在 2021/8/13 下午1:53, Jiri Slaby 写道:
> Hi,
>
> On 12. 08. 21, 14:26, kernel test robot wrote:
>>>> drivers/tty/hvc/hvc_console.c:190:26: warning: variable 'hp' is 
>>>> uninitialized when used here [-Wuninitialized]
>>             spin_unlock_irqrestore(&hp->c_lock, flags);
>>                                     ^~
>>     drivers/tty/hvc/hvc_console.c:149:23: note: initialize the 
>> variable 'hp' to silence this warning
>>             struct hvc_struct *hp;
>>                                  ^
>>                                   = NULL
>
> So you clearly didn't test your change as it would crash quite 
> instantly. I wonder, where do you intend to get hp from in the 
> console::print() hook?
>
> thanks,
According to analysis, this issue may can be solved just by adjust the 
alignment to L1_CACHE_BYTES: 

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

Our analysis as below, the original __ALIGNED__ is sizeof(long) which is 
8 for 64bit cpu.
char c[N_OUTBUF] __ALIGNED__; //c[16] __ALIGNED__;

For 4K page, c[16] may cross the page when alignemnt is 8.

In the case the physical address of c[16] is noncontiguous.
|------8----|..........|-----8-----|    PAGE_1
..........................|-----------16----------| c[16]

.........................................|-----8-----|.............|-----8-----| 
PAGE_2
But when the alignment is L1_CACHE_BYTES(eg, 64), or at least 
N_OUTBUF(16), we have no dma issue as c[16] won't cross the page, the 
physical address of c[16] is contiguous.
|--------64--------|.........|--------64---------| PAGE_3

..................................|--c[16]--|

Could you help comments this?  thanks

  parent reply	other threads:[~2021-08-13 16:07 UTC|newest]

Thread overview: 24+ 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 ` 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  9:45   ` Xianting Tian
2021-08-12 12:26   ` kernel test robot
2021-08-12 12:26     ` kernel test robot
2021-08-12 12:26     ` kernel test robot
2021-08-12 12:26     ` kernel test robot
2021-08-13  5:53     ` Jiri Slaby
2021-08-13  5:53       ` Jiri Slaby
2021-08-13  5:53       ` Jiri Slaby
2021-08-13  6:04       ` Xianting TIan
2021-08-13  6:04         ` Xianting TIan
2021-08-13  6:04         ` Xianting TIan
2021-08-13 16:07       ` Xianting TIan [this message]
2021-08-13 16:07         ` Xianting TIan
2021-08-13 16:07         ` Xianting TIan
2021-08-13  7:27   ` Greg KH
2021-08-13  7:27     ` Greg KH
2021-08-13  7:27     ` Greg KH
2021-08-13 11:27     ` Xianting TIan
2021-08-13 11:27       ` Xianting TIan
2021-08-12  9:45 ` [PATCH v6 2/2] virtio-console: remove unnecessary kmemdup() Xianting Tian
2021-08-12  9:45   ` Xianting Tian

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