Xen devel - Jul 2008 - Re: [Qemu-devel] [PATCH 2/2] open ptys in non-blocking mode.

[Crossposting to xen-devel]

Ian, we need something like this for qemu-xen (or ioemu-remote or
whatever it''s called now). Currently you must attach to the console of
a
domain, otherwise it won''t boot up and keep hanging in a blocking write
because the buffer is full.

The old ioemu had a hack in unix_write (doing a select before the write)
which you didn''t merge into qemu-xen. In fact, I noticed that you even
removed that function entirely and I''m wondering why.

Kevin

Gerd Hoffmann schrieb:
> Otherwise qemu will hang in case nobody connects to the pty and the
> guests prints enougth messages to fill up the buffer (which is 4k
> in linux).
> 
> Downside is that data may get lost in case the reader is too slow.
> 
> Ideally we could detect whenever someone is connected to the other end
> of the pseudo tty and write data only in connected mode (like it is done
> for tcp/telnet).  I''m not aware of any way to accomplish that
though.
> 
> Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
> ---
>  vl.c |    1 +
>  1 files changed, 1 insertions(+), 0 deletions(-)
> 
> diff --git a/vl.c b/vl.c
> index 587b91c..adc8f5f 100644
> --- a/vl.c
> +++ b/vl.c
> @@ -2468,6 +2468,7 @@ static CharDriverState *qemu_chr_open_pty(void)
>      /* Set raw attributes on the pty. */
>      cfmakeraw(&tty);
>      tcsetattr(slave_fd, TCSAFLUSH, &tty);
> +    socket_set_nonblock(master_fd);
>  
>      fprintf(stderr, "char device redirected to %s\n",
ptsname(master_fd));
>      return qemu_chr_open_fd(master_fd, master_fd);

_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xensource.com
http://lists.xensource.com/xen-devel

Kevin Wolf wrote:
> [Crossposting to xen-devel]
> 
> Ian, we need something like this for qemu-xen (or ioemu-remote or
> whatever it''s called now). Currently you must attach to the
console of a
> domain, otherwise it won''t boot up and keep hanging in a blocking
write
> because the buffer is full.
> 
> The old ioemu had a hack in unix_write (doing a select before the write)
> which you didn''t merge into qemu-xen. In fact, I noticed that you
even
> removed that function entirely and I''m wondering why.
For completeness:  You also need the attached patch for unix_write,
otherwise you''ll end up with qemu burning cpu cycles.  If you
can''t
write to a non-blocking file handle the write will instantly return with
-EAGAIN.  Calling it again of course doesn''t change the result, so
better don''t do that ...

-- 
http://kraxel.fedorapeople.org/xenner/


_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xensource.com
http://lists.xensource.com/xen-devel

Gerd Hoffmann schrieb:
> Kevin Wolf wrote:
>> [Crossposting to xen-devel]
>>
>> Ian, we need something like this for qemu-xen (or ioemu-remote or
>> whatever it''s called now). Currently you must attach to the
console of a
>> domain, otherwise it won''t boot up and keep hanging in a
blocking write
>> because the buffer is full.
>>
>> The old ioemu had a hack in unix_write (doing a select before the
write)
>> which you didn''t merge into qemu-xen. In fact, I noticed that
you even
>> removed that function entirely and I''m wondering why.
> 
> For completeness:  You also need the attached patch for unix_write,
> otherwise you''ll end up with qemu burning cpu cycles.  If you
can''t
> write to a non-blocking file handle the write will instantly return with
> -EAGAIN.  Calling it again of course doesn''t change the result, so
> better don''t do that ...
As I metioned above, in qemu-xen there is no unix_write. It calls
write() directly instead. But if we wanted to re-introduce unix_write to
be closer to upstream qemu (I still don''t know why unix_write was
dropped in the first place for qemu-xen - Ian?) you''re right, of
course.

Kevin

_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xensource.com
http://lists.xensource.com/xen-devel

Kevin Wolf writes ("Re: [Qemu-devel] [PATCH 2/2] open ptys in non-blocking
mode."):
> Ian, we need something like this for qemu-xen (or ioemu-remote or
> whatever it''s called now). Currently you must attach to the
console of a
> domain, otherwise it won''t boot up and keep hanging in a blocking
write
> because the buffer is full.
Yes, that''s right.  See my other messages, including the patches which
will be arriving shortly ...
> The old ioemu had a hack in unix_write (doing a select before the write)
> which you didn''t merge into qemu-xen. In fact, I noticed that you
even
> removed that function entirely and I''m wondering why.
We have qemu_{read,write} instead, as I wrote in January.  Those make
it easy to have good error handling.  Sadly my patch from January
wasn''t accepted upstream.

Ian.

_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xensource.com
http://lists.xensource.com/xen-devel