Re: [Qemu-devel] [PATCH 3/6] migration: add UFFD_FEATURE_THREAD_ID feature support

[Alexey <a.perevalov@samsung.com>]

On Fri, Apr 21, 2017 at 11:24:54AM +0100, Dr. David Alan Gilbert wrote:
> * Alexey Perevalov (a.perevalov@samsung.com) wrote:
> > Userfaultfd mechanism is able to provide process thread id,
> > in case when client request it with UFDD_API ioctl.
> > 
> > Signed-off-by: Alexey Perevalov <a.perevalov@samsung.com>
> 
> There seem to be two parts to this:
>   a) Adding the mis parameter to ufd_version_check
>   b) Asking for the feature
> 
> Please split it into two patches.
> 
> Also....
> 
> > ---
> >  include/migration/postcopy-ram.h |  2 +-
> >  migration/migration.c            |  2 +-
> >  migration/postcopy-ram.c         | 12 ++++++------
> >  migration/savevm.c               |  2 +-
> >  4 files changed, 9 insertions(+), 9 deletions(-)
> > 
> > diff --git a/include/migration/postcopy-ram.h b/include/migration/postcopy-ram.h
> > index 8e036b9..809f6db 100644
> > --- a/include/migration/postcopy-ram.h
> > +++ b/include/migration/postcopy-ram.h
> > @@ -14,7 +14,7 @@
> >  #define QEMU_POSTCOPY_RAM_H
> >  
> >  /* Return true if the host supports everything we need to do postcopy-ram */
> > -bool postcopy_ram_supported_by_host(void);
> > +bool postcopy_ram_supported_by_host(MigrationIncomingState *mis);
> >  
> >  /*
> >   * Make all of RAM sensitive to accesses to areas that haven't yet been written
> > diff --git a/migration/migration.c b/migration/migration.c
> > index ad4036f..79f6425 100644
> > --- a/migration/migration.c
> > +++ b/migration/migration.c
> > @@ -802,7 +802,7 @@ void qmp_migrate_set_capabilities(MigrationCapabilityStatusList *params,
> >           * special support.
> >           */
> >          if (!old_postcopy_cap && runstate_check(RUN_STATE_INMIGRATE) &&
> > -            !postcopy_ram_supported_by_host()) {
> > +            !postcopy_ram_supported_by_host(NULL)) {
> >              /* postcopy_ram_supported_by_host will have emitted a more
> >               * detailed message
> >               */
> > diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
> > index dc80dbb..70f0480 100644
> > --- a/migration/postcopy-ram.c
> > +++ b/migration/postcopy-ram.c
> > @@ -60,13 +60,13 @@ struct PostcopyDiscardState {
> >  #include <sys/eventfd.h>
> >  #include <linux/userfaultfd.h>
> >  
> > -static bool ufd_version_check(int ufd)
> > +static bool ufd_version_check(int ufd, MigrationIncomingState *mis)
> >  {
> >      struct uffdio_api api_struct;
> >      uint64_t ioctl_mask;
> >  
> >      api_struct.api = UFFD_API;
> > -    api_struct.features = 0;
> > +    api_struct.features = UFFD_FEATURE_THREAD_ID;
> >      if (ioctl(ufd, UFFDIO_API, &api_struct)) {
> >          error_report("postcopy_ram_supported_by_host: UFFDIO_API failed: %s",
> >                       strerror(errno));
> 
> You're not actually using the 'mis' here - what I'd expected was
> something that was going to check if the UFFDIO_API return said that it really
> had the feature, and if so store a flag in the MIS somewhere.
> 
> Also, I'm not sure it's right to set 'api_struct.features' on the input - what
> happens if this is run on an old kernel - we don't want postcopy to fail on
> an old kernel without your feature.
> I'm not 100% sure of the interface, but I think the way it works is you set
> features = 0 before the call, and then check the api_struct.features in the
> return - in the same way that I check for UFFD_FEATURE_MISSING_HUGETLBFS.
> 
We need to ask kernel about that feature,
right,
kernel returns back available features
uffdio_api.features = UFFD_API_FEATURES
but it also stores requested features
/* only enable the requested features for this uffd context */
 ctx->features = uffd_ctx_features(features);

so, at the time when process thread id is going to be sent
kernel checks if it was requested
+       if (features & UFFD_FEATURE_THREAD_ID)
+               msg.arg.pagefault.ptid = task_pid_vnr(current);

from patch message:

 Process's thread id is being provided when user requeste it
by setting UFFD_FEATURE_THREAD_ID bit into uffdio_api.features.

UFFD_FEATURE_MISSING_HUGETLBFS - look like default, unconditional
behavior (I didn't find any usage of that define in kernel).


> Dave
> 
> > @@ -113,7 +113,7 @@ static int test_range_shared(const char *block_name, void *host_addr,
> >   * normally fine since if the postcopy succeeds it gets turned back on at the
> >   * end.
> >   */
> > -bool postcopy_ram_supported_by_host(void)
> > +bool postcopy_ram_supported_by_host(MigrationIncomingState *mis)
> >  {
> >      long pagesize = getpagesize();
> >      int ufd = -1;
> > @@ -136,7 +136,7 @@ bool postcopy_ram_supported_by_host(void)
> >      }
> >  
> >      /* Version and features check */
> > -    if (!ufd_version_check(ufd)) {
> > +    if (!ufd_version_check(ufd, mis)) {
> >          goto out;
> >      }
> >  
> > @@ -515,7 +515,7 @@ int postcopy_ram_enable_notify(MigrationIncomingState *mis)
> >       * Although the host check already tested the API, we need to
> >       * do the check again as an ABI handshake on the new fd.
> >       */
> > -    if (!ufd_version_check(mis->userfault_fd)) {
> > +    if (!ufd_version_check(mis->userfault_fd, mis)) {
> >          return -1;
> >      }
> >  
> > @@ -653,7 +653,7 @@ void *postcopy_get_tmp_page(MigrationIncomingState *mis)
> >  
> >  #else
> >  /* No target OS support, stubs just fail */
> > -bool postcopy_ram_supported_by_host(void)
> > +bool postcopy_ram_supported_by_host(MigrationIncomingState *mis)
> >  {
> >      error_report("%s: No OS support", __func__);
> >      return false;
> > diff --git a/migration/savevm.c b/migration/savevm.c
> > index 3b19a4a..f01e418 100644
> > --- a/migration/savevm.c
> > +++ b/migration/savevm.c
> > @@ -1360,7 +1360,7 @@ static int loadvm_postcopy_handle_advise(MigrationIncomingState *mis)
> >          return -1;
> >      }
> >  
> > -    if (!postcopy_ram_supported_by_host()) {
> > +    if (!postcopy_ram_supported_by_host(mis)) {
> >          postcopy_state_set(POSTCOPY_INCOMING_NONE);
> >          return -1;
> >      }
> > -- 
> > 1.8.3.1
> > 
> --
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> 

-- 

BR
Alexey