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

[Alexey]

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.

> 
> 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