Re: [Xen-devel] [PATCH v4] libxl: reimplement buffer for bootloading and drop data if buffer is full

Roger Pau Monné
Wed, 19 Oct 2011 07:42:57 -0000

2011/10/18 Ian Campbell <Ian.Campbell@citrix.com>:
> On Tue, 2011-10-18 at 14:38 +0100, Roger Pau Monne wrote:
>> # HG changeset patch
>> # User Roger Pau Monne <roger.pau@entel.upc.edu>
>> # Date 1318944898 -7200
>> # Node ID f7b311b85973f5862c323fed4e89c20b7af139a8
>> # Parent  6fd16bcdd3e55bf8cc6e6e90e910f32e37654fd7
>> libxl: reimplement buffer for bootloading and drop data if buffer is
full.
>>
>> Implement a buffer for the bootloading process that appends data to the
end until it''s full. Drop the whole buffer if a timeout has occurred
and the buffer is full. Prevents the bootloader from getting stuck when using
ptys with small buffers.
>>
>> Signed-off-by: Roger Pau Monne <roger.pau@entel.upc.edu>
>>
>> diff -r 6fd16bcdd3e5 -r f7b311b85973 tools/libxl/libxl_bootloader.c
>> --- a/tools/libxl/libxl_bootloader.c  Tue Oct 18 15:10:04 2011 +0200
>> +++ b/tools/libxl/libxl_bootloader.c  Tue Oct 18 15:34:58 2011 +0200
>> @@ -28,7 +28,8 @@
>>  #include "flexarray.h"
>>
>>  #define XENCONSOLED_BUF_SIZE 16
>> -#define BOOTLOADER_BUF_SIZE 1024
>> +#define BOOTLOADER_BUF_SIZE 4096
>> +#define BOOTLOADER_TIMEOUT 1
>>
>>  static char **make_bootloader_args(libxl__gc *gc,
>>                                     libxl_domain_build_info *info,
>> @@ -169,6 +170,7 @@ static char * bootloader_interact(libxl_
>>
>>      size_t nr_out = 0, size_out = 0;
>>      char *output = NULL;
>> +    struct timeval wait;
>>
>>      /* input from xenconsole. read on xenconsoled_fd write to
bootloader_fd */
>>      int xenconsoled_prod = 0, xenconsoled_cons = 0;
>> @@ -181,39 +183,66 @@ static char * bootloader_interact(libxl_
>>          fd_set wsel, rsel;
>>          int nfds;
>>
>> +        /* Set timeout to 1s before starting to discard data */
>> +        wait.tv_sec = BOOTLOADER_TIMEOUT;
>> +        wait.tv_usec = 0;
>> +
>>          if (xenconsoled_prod == xenconsoled_cons)
>>              xenconsoled_prod = xenconsoled_cons = 0;
>>          if (bootloader_prod == bootloader_cons)
>>              bootloader_prod = bootloader_cons = 0;

Now that I also look at it, I think this "ifs" could be removed, the
following checks for cons > 0 will perform the same task if cons =prod, and
the code will look cleaner.

>> +        /* Move buffers around to drop already consumed data */
>> +        if (xenconsoled_cons > 0) {
>> +            xenconsoled_prod -= xenconsoled_cons;
>> +            memmove(xenconsoled_buf,
&xenconsoled_buf[xenconsoled_cons],
>> +                    xenconsoled_prod);
>> +            xenconsoled_cons = 0;
>> +        }
>> +        if (bootloader_cons > 0) {
>> +            bootloader_prod -= bootloader_cons;
>> +            memmove(bootloader_buf,
&bootloader_buf[bootloader_cons],
>> +                    bootloader_prod);
>> +            bootloader_cons = 0;
>> +        }
>>
>>          FD_ZERO(&rsel);
>>          FD_SET(fifo_fd, &rsel);
>>          nfds = fifo_fd + 1;
>> -        if (xenconsoled_prod == 0 || (xenconsoled_prod <
BOOTLOADER_BUF_SIZE && xenconsoled_cons == 0)) {
>> +        if (xenconsoled_prod < XENCONSOLED_BUF_SIZE) {
>>              FD_SET(xenconsoled_fd, &rsel);
>>              nfds = xenconsoled_fd + 1 > nfds ? xenconsoled_fd + 1 :
nfds;
>> -        }
>> -        if (bootloader_prod == 0 || (bootloader_prod <
BOOTLOADER_BUF_SIZE && bootloader_cons == 0)) {
>> +        }
>> +        if (bootloader_prod < BOOTLOADER_BUF_SIZE) {
>>              FD_SET(bootloader_fd, &rsel);
>>              nfds = bootloader_fd + 1 > nfds ? bootloader_fd + 1 :
nfds;
>>          }
>>
>>          FD_ZERO(&wsel);
>> -        if (bootloader_prod != bootloader_cons) {
>> +        if (bootloader_prod > 0) {
>
> It''s a little less obvious now that this test means "there is
data to
> consume", a comment might be appropriate.

Added a couple of comments.

>>              FD_SET(xenconsoled_fd, &wsel);
>>              nfds = xenconsoled_fd + 1 > nfds ? xenconsoled_fd + 1 :
nfds;
>>          }
>> -        if (xenconsoled_prod != xenconsoled_cons) {
>> +        if (xenconsoled_prod > 0) {
>>              FD_SET(bootloader_fd, &wsel);
>>              nfds = bootloader_fd + 1 > nfds ? bootloader_fd + 1 :
nfds;
>>          }
>>
>> -        ret = select(nfds, &rsel, &wsel, NULL, NULL);
>> -        if (ret < 0)
>> +        if (xenconsoled_prod == XENCONSOLED_BUF_SIZE ||
>> +            bootloader_prod == BOOTLOADER_BUF_SIZE)
>> +            ret = select(nfds, &rsel, &wsel, NULL, &wait);
>> +        else
>> +            ret = select(nfds, &rsel, &wsel, NULL, NULL);
>> +        if (ret < 0) {
>> +            if (errno == EINTR)
>> +                continue;
>>              goto out_err;
>> +        }
>>
>>          /* Input from xenconsole, read xenconsoled_fd, write
bootloader_fd */
>> -        if (FD_ISSET(xenconsoled_fd, &rsel)) {
>> +        if (ret == 0 && xenconsoled_prod ==
XENCONSOLED_BUF_SIZE) {
>> +            /* Drop the buffer */
>> +            xenconsoled_prod = 0;
>
> Do you also need to reset cons here? I expect we could prove it was
> always zero anyway but it might be more obvious to just reset it.

It is always 0 because we check "cons > 0" at the start of the
loop,
and set it to 0 if it is bigger. If you feel it''s best to set it to
zero here to make the code easier to understand it''s fine for me.

> Otherwise this looks good, thanks.

Thanks to you and Ian Jackson for reviewing the code and having so
much patience.

>> +        } else if (FD_ISSET(xenconsoled_fd, &rsel)) {
>>              ret = read(xenconsoled_fd,
&xenconsoled_buf[xenconsoled_prod], XENCONSOLED_BUF_SIZE -
xenconsoled_prod);
>>              if (ret < 0 && errno != EIO && errno !=
EAGAIN)
>>                  goto out_err;
>> @@ -229,7 +258,10 @@ static char * bootloader_interact(libxl_
>>          }
>>
>>          /* Input from bootloader, read bootloader_fd, write
xenconsoled_fd */
>> -        if (FD_ISSET(bootloader_fd, &rsel)) {
>> +        if (ret == 0 && bootloader_prod ==
BOOTLOADER_BUF_SIZE) {
>> +            /* Drop the buffer */
>> +            bootloader_prod = 0;
>> +        } else if (FD_ISSET(bootloader_fd, &rsel)) {
>>              ret = read(bootloader_fd,
&bootloader_buf[bootloader_prod], BOOTLOADER_BUF_SIZE - bootloader_prod);
>>              if (ret < 0 && errno != EIO && errno !=
EAGAIN)
>>                  goto out_err;
>>
>> _______________________________________________
>> Xen-devel mailing list
>> Xen-devel@lists.xensource.com
>> http://lists.xensource.com/xen-devel