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 (a.pereva...@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.pereva...@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
>
> 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
