Re: [PATCH V5 20/25] chardev: cpr framework

[Steven Sistare]

On 7/8/2021 12:03 PM, Marc-André Lureau wrote:
> Hi
> 
> On Wed, Jul 7, 2021 at 9:37 PM Steve Sistare <steven.sistare@oracle.com 
> <mailto:steven.sistare@oracle.com>> wrote:
> 
>     Add QEMU_CHAR_FEATURE_CPR for devices that support cpr.
>     Add the chardev close_on_cpr option for devices that can be closed on cpr
>     and reopened after exec.
>     cpr is allowed only if either QEMU_CHAR_FEATURE_CPR or close_on_cpr is set
>     for all chardevs in the configuration.
> 
> 
> Why not do the right thing by default?

Char devices with buffering in the qemu process do not support cpr, as there is 
no general mechanism 
for saving and restoring the buffer and synchronizing that with device 
operation.  In theory vmstate 
could provide that mechanism, but sync'ing the device with vmstate operations 
would be non-trivial, 
as every device handles it differently, and I did not tackle it.  However, some 
very  useful devices 
do not buffer, and do support cpr, so I introduce QEMU_CHAR_FEATURE_CPR to 
identify them.  CPR support
can be incrementally added to more devices in the future via this mechanism.

> Could use some tests in tests/unit/test-char.c

OK, I'll check it out.  I have deferred adding unit tests until I get more buy 
in on the patch series.

>     Signed-off-by: Steve Sistare <steven.sistare@oracle.com 
> <mailto:steven.sistare@oracle.com>>
>     ---
>      chardev/char.c         | 41 ++++++++++++++++++++++++++++++++++++++---
>      include/chardev/char.h |  5 +++++
>      migration/cpr.c        |  3 +++
>      qapi/char.json         |  5 ++++-
>      qemu-options.hx        | 26 ++++++++++++++++++++++----
>      5 files changed, 72 insertions(+), 8 deletions(-)
> 
>     diff --git a/chardev/char.c b/chardev/char.c
>     index d959eec..f10fb94 100644
>     --- a/chardev/char.c
>     +++ b/chardev/char.c
>     @@ -36,6 +36,7 @@
>      #include "qemu/help_option.h"
>      #include "qemu/module.h"
>      #include "qemu/option.h"
>     +#include "qemu/env.h"
>      #include "qemu/id.h"
>      #include "qemu/coroutine.h"
>      #include "qemu/yank.h"
>     @@ -239,6 +240,9 @@ static void qemu_char_open(Chardev *chr, 
> ChardevBackend *backend,
>          ChardevClass *cc = CHARDEV_GET_CLASS(chr);
>          /* Any ChardevCommon member would work */
>          ChardevCommon *common = backend ? backend->u.null.data : NULL;
>     +    char fdname[40];
> 
> 
> Please use g_autoptr char *fdname = NULL; & g_strdup_printf()

Will do.  
(the glibc functions are new to me, and my fingers do not automatically type 
them).

>     +
>     +    chr->close_on_cpr = (common && common->close_on_cpr);
> 
>          if (common && common->has_logfile) {
>              int flags = O_WRONLY | O_CREAT;
>     @@ -248,7 +252,14 @@ static void qemu_char_open(Chardev *chr, 
> ChardevBackend *backend,
>              } else {
>                  flags |= O_TRUNC;
>              }
>     -        chr->logfd = qemu_open_old(common->logfile, flags, 0666);
>     +        snprintf(fdname, sizeof(fdname), "%s_log", chr->label);
>     +        chr->logfd = getenv_fd(fdname);
>     +        if (chr->logfd < 0) {
>     +            chr->logfd = qemu_open_old(common->logfile, flags, 0666);
>     +            if (!chr->close_on_cpr) {
>     +                setenv_fd(fdname, chr->logfd);
>     +            }
>     +        }
>              if (chr->logfd < 0) {
>                  error_setg_errno(errp, errno,
>                                   "Unable to open logfile %s",
>     @@ -300,11 +311,12 @@ static void char_finalize(Object *obj)
>          if (chr->be) {
>              chr->be->chr = NULL;
>          }
>     -    g_free(chr->filename);
>     -    g_free(chr->label);
>          if (chr->logfd != -1) {
>              close(chr->logfd);
>     +        unsetenv_fdv("%s_log", chr->label);
>          }
>     +    g_free(chr->filename);
>     +    g_free(chr->label);
>          qemu_mutex_destroy(&chr->chr_write_lock);
>      }
> 
>     @@ -504,6 +516,8 @@ void qemu_chr_parse_common(QemuOpts *opts, 
> ChardevCommon *backend)
> 
>          backend->has_logappend = true;
>          backend->logappend = qemu_opt_get_bool(opts, "logappend", false);
>     +
>     +    backend->close_on_cpr = qemu_opt_get_bool(opts, "close-on-cpr", 
> false);
> 
> 
> If set to true and the backend doesn't implement the CPR feature, it should 
> raise an error.

Setting to true is the workaround for missing CPR support, so that cpr may 
still be performed.  
The device will be reopened post exec.  That is not as nice as transparently 
preserving the device, 
but is nicer than disallowing cpr because some device(s) of many do not support 
it.

>      }
> 
>      static const ChardevClass *char_get_class(const char *driver, Error 
> **errp)
>     @@ -945,6 +959,9 @@ QemuOptsList qemu_chardev_opts = {
>              },{
>                  .name = "abstract",
>                  .type = QEMU_OPT_BOOL,
>     +        },{
>     +            .name = "close-on-cpr",
>     +            .type = QEMU_OPT_BOOL,
>      #endif
>              },
>              { /* end of list */ }
>     @@ -1212,6 +1229,24 @@ GSource *qemu_chr_timeout_add_ms(Chardev *chr, 
> guint ms,
>          return source;
>      }
> 
>     +static int chr_cpr_capable(Object *obj, void *opaque)
>     +{
>     +    Chardev *chr = (Chardev *)obj;
>     +    Error **errp = opaque;
>     +
>     +    if (qemu_chr_has_feature(chr, QEMU_CHAR_FEATURE_CPR) || 
> chr->close_on_cpr) {
> 
> 
> That'd be easy to misuse. Chardev should always explicitly support CPR 
> feature (even if close_on_cpr is set)

Given my explanation at top, does this make sense now?

- Steve


>     +        return 0;
>     +    }
>     +    error_setg(errp, "error: chardev %s -> %s is not capable of cpr",
>     +               chr->label, chr->filename);
>     +    return 1;
>     +}
>     +
>     +bool qemu_chr_cpr_capable(Error **errp)
>     +{
>     +    return !object_child_foreach(get_chardevs_root(), chr_cpr_capable, 
> errp);
>     +}
>     +
>      void qemu_chr_cleanup(void)
>      {
>          object_unparent(get_chardevs_root());
>     diff --git a/include/chardev/char.h b/include/chardev/char.h
>     index 7c0444f..e488ad1 100644
>     --- a/include/chardev/char.h
>     +++ b/include/chardev/char.h
>     @@ -50,6 +50,8 @@ typedef enum {
>          /* Whether the gcontext can be changed after calling
>           * qemu_chr_be_update_read_handlers() */
>          QEMU_CHAR_FEATURE_GCONTEXT,
>     +    /* Whether the device supports cpr */
>     +    QEMU_CHAR_FEATURE_CPR,
> 
>          QEMU_CHAR_FEATURE_LAST,
>      } ChardevFeature;
>     @@ -67,6 +69,7 @@ struct Chardev {
>          int be_open;
>          /* used to coordinate the chardev-change special-case: */
>          bool handover_yank_instance;
>     +    bool close_on_cpr;
>          GSource *gsource;
>          GMainContext *gcontext;
>          DECLARE_BITMAP(features, QEMU_CHAR_FEATURE_LAST);
>     @@ -291,4 +294,6 @@ void resume_mux_open(void);
>      /* console.c */
>      void qemu_chr_parse_vc(QemuOpts *opts, ChardevBackend *backend, Error 
> **errp);
> 
>     +bool qemu_chr_cpr_capable(Error **errp);
>     +
>      #endif
>     diff --git a/migration/cpr.c b/migration/cpr.c
>     index 6333988..feff97f 100644
>     --- a/migration/cpr.c
>     +++ b/migration/cpr.c
>     @@ -138,6 +138,9 @@ void cprexec(strList *args, Error **errp)
>              error_setg(errp, "cprexec requires cprsave with restart mode");
>              return;
>          }
>     +    if (!qemu_chr_cpr_capable(errp)) {
>     +        return;
>     +    }
>          if (vfio_cprsave(errp)) {
>              return;
>          }
>     diff --git a/qapi/char.json b/qapi/char.json
>     index adf2685..5efaf59 100644
>     --- a/qapi/char.json
>     +++ b/qapi/char.json
>     @@ -204,12 +204,15 @@
>      # @logfile: The name of a logfile to save output
>      # @logappend: true to append instead of truncate
>      #             (default to false to truncate)
>     +# @close-on-cpr: if true, close device's fd on cprsave. defaults to 
> false.
>     +#                since 6.1.
>      #
>      # Since: 2.6
>      ##
>      { 'struct': 'ChardevCommon',
>        'data': { '*logfile': 'str',
>     -            '*logappend': 'bool' } }
>     +            '*logappend': 'bool',
>     +            '*close-on-cpr': 'bool' } }
> 
>      ##
>      # @ChardevFile:
>     diff --git a/qemu-options.hx b/qemu-options.hx
>     index fa53734..d5ff45f 100644
>     --- a/qemu-options.hx
>     +++ b/qemu-options.hx
>     @@ -3134,43 +3134,57 @@ DEFHEADING(Character device options:)
> 
>      DEF("chardev", HAS_ARG, QEMU_OPTION_chardev,
>          "-chardev help\n"
>     -    "-chardev 
> null,id=id[,mux=on|off][,logfile=PATH][,logappend=on|off]\n"
>     +    "-chardev 
> null,id=id[,mux=on|off][,logfile=PATH][,logappend=on|off][,close-on-cpr=on|off]\n"
>          "-chardev 
> socket,id=id[,host=host],port=port[,to=to][,ipv4=on|off][,ipv6=on|off][,nodelay=on|off][,reconnect=seconds]\n"
>          "         
> [,server=on|off][,wait=on|off][,telnet=on|off][,websocket=on|off][,reconnect=seconds][,mux=on|off]\n"
>     -    "         
> [,logfile=PATH][,logappend=on|off][,tls-creds=ID][,tls-authz=ID] (tcp)\n"
>     +    "         
> [,logfile=PATH][,logappend=on|off][,tls-creds=ID][,tls-authz=ID][,close-on-cpr=on|off]
>  (tcp)\n"
>          "-chardev 
> socket,id=id,path=path[,server=on|off][,wait=on|off][,telnet=on|off][,websocket=on|off][,reconnect=seconds]\n"
>     -    "         
> [,mux=on|off][,logfile=PATH][,logappend=on|off][,abstract=on|off][,tight=on|off]
>  (unix)\n"
>     +    "         
> [,mux=on|off][,logfile=PATH][,logappend=on|off][,abstract=on|off][,tight=on|off][,close-on-cpr=on|off]
>  (unix)\n"
>          "-chardev udp,id=id[,host=host],port=port[,localaddr=localaddr]\n"
>          "         
> [,localport=localport][,ipv4=on|off][,ipv6=on|off][,mux=on|off]\n"
>     -    "         [,logfile=PATH][,logappend=on|off]\n"
>     +    "         [,logfile=PATH][,logappend=on|off][,close-on-cpr=on|off]\n"
>          "-chardev 
> msmouse,id=id[,mux=on|off][,logfile=PATH][,logappend=on|off]\n"
>     +    "         [,close-on-cpr=on|off]\n"
>          "-chardev 
> vc,id=id[[,width=width][,height=height]][[,cols=cols][,rows=rows]]\n"
>          "         [,mux=on|off][,logfile=PATH][,logappend=on|off]\n"
>     +    "         [,close-on-cpr=on|off]\n"
>          "-chardev 
> ringbuf,id=id[,size=size][,logfile=PATH][,logappend=on|off]\n"
>     +    "         [,close-on-cpr=on|off]\n"
>          "-chardev 
> file,id=id,path=path[,mux=on|off][,logfile=PATH][,logappend=on|off]\n"
>     +    "         [,close-on-cpr=on|off]\n"
>          "-chardev 
> pipe,id=id,path=path[,mux=on|off][,logfile=PATH][,logappend=on|off]\n"
>     +    "         [,close-on-cpr=on|off]\n"
>      #ifdef _WIN32
>          "-chardev 
> console,id=id[,mux=on|off][,logfile=PATH][,logappend=on|off]\n"
>          "-chardev 
> serial,id=id,path=path[,mux=on|off][,logfile=PATH][,logappend=on|off]\n"
>      #else
>          "-chardev pty,id=id[,mux=on|off][,logfile=PATH][,logappend=on|off]\n"
>     +    "         [,close-on-cpr=on|off]\n"
>          "-chardev 
> stdio,id=id[,mux=on|off][,signal=on|off][,logfile=PATH][,logappend=on|off]\n"
>     +    "         [,close-on-cpr=on|off]\n"
>      #endif
>      #ifdef CONFIG_BRLAPI
>          "-chardev 
> braille,id=id[,mux=on|off][,logfile=PATH][,logappend=on|off]\n"
>     +    "         [,close-on-cpr=on|off]\n"
>      #endif
>      #if defined(__linux__) || defined(__sun__) || defined(__FreeBSD__) \
>              || defined(__NetBSD__) || defined(__OpenBSD__) || 
> defined(__DragonFly__)
>          "-chardev 
> serial,id=id,path=path[,mux=on|off][,logfile=PATH][,logappend=on|off]\n"
>     +    "         [,close-on-cpr=on|off]\n"
>          "-chardev 
> tty,id=id,path=path[,mux=on|off][,logfile=PATH][,logappend=on|off]\n"
>     +    "         [,close-on-cpr=on|off]\n"
>      #endif
>      #if defined(__linux__) || defined(__FreeBSD__) || defined(__DragonFly__)
>          "-chardev 
> parallel,id=id,path=path[,mux=on|off][,logfile=PATH][,logappend=on|off]\n"
>     +    "         [,close-on-cpr=on|off]\n"
>          "-chardev 
> parport,id=id,path=path[,mux=on|off][,logfile=PATH][,logappend=on|off]\n"
>     +    "         [,close-on-cpr=on|off]\n"
>      #endif
>      #if defined(CONFIG_SPICE)
>          "-chardev 
> spicevmc,id=id,name=name[,debug=debug][,logfile=PATH][,logappend=on|off]\n"
>     +    "         [,close-on-cpr=on|off]\n"
>          "-chardev 
> spiceport,id=id,name=name[,debug=debug][,logfile=PATH][,logappend=on|off]\n"
>     +    "         [,close-on-cpr=on|off]\n"
>      #endif
>          , QEMU_ARCH_ALL
>      )
>     @@ -3245,6 +3259,10 @@ The general form of a character device option is:
>          ``logappend`` option controls whether the log file will be truncated
>          or appended to when opened.
> 
>     +    Every backend supports the ``close-on-cpr`` option.  If on, the
>     +    devices's descriptor is closed during cprsave, and reopened after 
> exec.
>     +    This is useful for devices that do not support cpr.
>     +
>      The available backends are:
> 
>      ``-chardev null,id=id``
>     -- 
>     1.8.3.1
> 
> 
> 
> 
> -- 
> Marc-André Lureau