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

[Dr. David Alan Gilbert]

* Alexey (address@hidden) wrote:
> On Mon, Apr 24, 2017 at 04:12:29PM +0800, Peter Xu wrote:
> > On Fri, Apr 21, 2017 at 06:22:12PM +0300, Alexey wrote:
> > > On Fri, Apr 21, 2017 at 11:24:54AM +0100, Dr. David Alan Gilbert wrote:
> > > > * Alexey Perevalov (address@hidden) 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 <address@hidden>
> > > > 
> > > > 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
> > 
> > I feel like this does not against Dave's comment, maybe we just need
> > to send the UFFDIO_API twice? Like:
> yes, ioctl with UFFDIO_API will fail on old kernel if we will request
> e.g. UFFD_FEATURE_THREAD_ID or other new feature.
> 
> So in general way need a per feature request, for better error handling.

No, we don't need to - I think the way the kernel works is that you pass
features = 0 in, and it sets api_struct.features on the way out;
so if you always pass 0 in, you can then just check the features that
it returns.

Dave

> 
> > 
> > diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
> > index 85fd8d7..fd0905f 100644
> > --- a/migration/postcopy-ram.c
> > +++ b/migration/postcopy-ram.c
> > @@ -64,6 +64,7 @@ static bool ufd_version_check(int ufd)
> >  {
> >      struct uffdio_api api_struct;
> >      uint64_t ioctl_mask;
> > +    uint64_t features = 0;
> > 
> >      api_struct.api = UFFD_API;
> >      api_struct.features = 0;
> > @@ -92,6 +93,27 @@ static bool ufd_version_check(int ufd)
> >              return false;
> >          }
> >      }
> > +
> > +#ifdef UFFD_FEATURE_THREAD_ID
> > +    if (api_struct.features & UFFD_FEATURE_THREAD_ID) {
> > +        features |= UFFD_FEATURE_THREAD_ID;
> > +    }
> > +#endif
> > +
> > +    if (features) {
> > +        /*
> > +         * If there are new features to be enabled from userspace,
> > +         * trigger another UFFDIO_API ioctl.
> > +         */
> > +        api_struct.api = UFFD_API;
> > +        api_struct.features = features;
> > +        if (ioctl(ufd, UFFDIO_API, &api_struct)) {
> > +            error_report("UFFDIO_API failed to setup features: 0x%"PRIx64,
> > +                         features);
> > +            return false;
> > +        }
> > +    }
> > +
> >      return true;
> >  }
> > 
> > > /* 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);
> > 
> > (I am slightly curious about why we need this if block, after all
> >  userspace should know whether the ptid field would be valid from the
> >  first UFFDIO_API ioctl, right?)
> If I correctly understand you question ) that condition was suggested,
> due to page faulting is performance critical part (in general, not only 
> postcopy
> case ), that's why it should be enabled from userspace, 
> only for statistics/debug purpose.
> Also looks like David want to see that feature on QEMU as not always
> feature too.
> 
> > 
> > Thanks,
> > 
> > > 
> > > 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).
> > 
> > -- 
> > Peter Xu
> > 
> 
> -- 
> 
> BR
> Alexey
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK