[uml-devel] Clarification of UML and AIO?

[uml-devel] Clarification of UML and AIO?

[Allan Graves]

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Hi guys...  Trying to do some AIO stuff here, and got a question.  
Perhaps I'm not understanding how AIO works in UML...  Could someone 
perhaps give me an explanation of why we end up in pread, instead of 
aio?  Does UML not support AIO through its block device driver?
Thank you,
Allan

The simple test program:

#include <aio.h>
#include <errno.h>
#include <fcntl.h>

main()
{
       struct aiocb aiocb;
       int fd;
       char buf[1024];

       fd = open("/tmp/outfile", O_RDONLY);
       perror("open");

       bzero(&aiocb, sizeof(aiocb));
       aiocb.aio_fildes = fd;
       aiocb.aio_buf = buf;
       aiocb.aio_nbytes = sizeof(buf);

       aio_read(&aiocb);
       perror("aio_read");

       while (aio_error(&aiocb) == EINPROGRESS);
       printf("buf = %s\n", buf);
}

Tracing the aio_read() through the UML kernel finally gets me here:

#0  sys_pread64 (fd=3, buf=0xbf7c8f20 "", count=1024, pos=0)
   at fs/read_write.c:354
#1  0x08062ff8 in execute_syscall_skas (r=0x12374864)
   at arch/um/kernel/skas/syscall_kern.c:29
#2  0x0806302f in handle_syscall (regs=0x12374864)
   at arch/um/kernel/skas/syscall_user.c:25
#3  0x08062651 in userspace (regs=0x12374864)
   at arch/um/kernel/skas/process.c:91
#4  0x08062d44 in fork_handler (sig=10) at thread_info.h:47
#5  <signal handler called>
#6  0xffffe410 in __kernel_vsyscall ()
#7  0x00000000 in ?? ()
(gdb)


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Re: [uml-devel] Clarification of UML and AIO?

[Jeff Dike]

> Tracing the aio_read() through the UML kernel finally gets me here:
> 
> #0  sys_pread64 (fd=3, buf=0xbf7c8f20 "", count=1024, pos=0)
>   at fs/read_write.c:354
> #1  0x08062ff8 in execute_syscall_skas (r=0x12374864)
>   at arch/um/kernel/skas/syscall_kern.c:29

Finally?  That's the very beginning of the system call (or the very end).

The UML block device doesn't know about AIO.  All it does is get blocks from
a disk.

Are you sure you're getting kernel AIO and not the libc emulation of it?

				Jeff


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Re: [uml-devel] Clarification of UML and AIO?

[Blaisorblade]

On Wednesday 13 July 2005 00:03, Jeff Dike wrote:
> > Tracing the aio_read() through the UML kernel finally gets me here:
> >
> > #0  sys_pread64 (fd=3, buf=0xbf7c8f20 "", count=1024, pos=0)
> >   at fs/read_write.c:354
> > #1  0x08062ff8 in execute_syscall_skas (r=0x12374864)
> >   at arch/um/kernel/skas/syscall_kern.c:29

> Finally?  That's the very beginning of the system call (or the very end).

> The UML block device doesn't know about AIO.  All it does is get blocks
> from a disk.

> Are you sure you're getting kernel AIO and not the libc emulation of it?
This is a good answer.... strace will be your friend in verifying this.
-- 
Inform me of my mistakes, so I can keep imitating Homer Simpson's "Doh!".
Paolo Giarrusso, aka Blaisorblade (Skype ID "PaoloGiarrusso", ICQ 215621894)
http://www.user-mode-linux.org/~blaisorblade

	

	
		
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Re: [uml-devel] Clarification of UML and AIO?

[Allan Graves]

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Jeff, Blaisor, you're exactly right.  Linux aio uses io_submit, not 
aio_read, it seems.  aio_read calls clone to make a new thread to do its 
"aio".  Thank you all for your help.
Allan

Jeff Dike wrote:

>>Tracing the aio_read() through the UML kernel finally gets me here:
>>
>>#0  sys_pread64 (fd=3, buf=0xbf7c8f20 "", count=1024, pos=0)
>>  at fs/read_write.c:354
>>#1  0x08062ff8 in execute_syscall_skas (r=0x12374864)
>>  at arch/um/kernel/skas/syscall_kern.c:29
>>    
>>
>
>Finally?  That's the very beginning of the system call (or the very end).
>
>The UML block device doesn't know about AIO.  All it does is get blocks from
>a disk.
>
>Are you sure you're getting kernel AIO and not the libc emulation of it?
>
>				Jeff
>  
>

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