Re: [Qemu-devel] [PATCH V5 4/9] migration: split ufd_version_check onto receive/request features part

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

On Tue, May 16, 2017 at 11:32:51AM +0100, Dr. David Alan Gilbert wrote:
> * Alexey Perevalov (a.perevalov@samsung.com) wrote:
> > This modification is necessary for userfault fd features which are
> > required to be requested from userspace.
> > UFFD_FEATURE_THREAD_ID is a one of such "on demand" feature, which will
> > be introduced in the next patch.
> > 
> > QEMU need to use separate userfault file descriptor, due to
> > userfault context has internal state, and after first call of
> > ioctl UFFD_API it changes its state to UFFD_STATE_RUNNING (in case of
> > success), but
> > kernel while handling ioctl UFFD_API expects UFFD_STATE_WAIT_API. So
> > only one ioctl with UFFD_API is possible per ufd.
> > 
> > Signed-off-by: Alexey Perevalov <a.perevalov@samsung.com>
> > ---
> >  migration/postcopy-ram.c | 82 ++++++++++++++++++++++++++++++++++++++++++------
> >  1 file changed, 73 insertions(+), 9 deletions(-)
> > 
> > diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
> > index 0f75700..c96d5f5 100644
> > --- a/migration/postcopy-ram.c
> > +++ b/migration/postcopy-ram.c
> > @@ -60,32 +60,96 @@ struct PostcopyDiscardState {
> >  #include <sys/eventfd.h>
> >  #include <linux/userfaultfd.h>
> >  
> > -static bool ufd_version_check(int ufd, MigrationIncomingState *mis)
> > +
> > +/*
> > + * Check userfault fd features, to request only supported features in
> > + * future.
> > + * __NR_userfaultfd - should be checked before
> > + * Return obtained features
> 
> That's not quite right;
>  * Returns: True on success, sets *features to supported features
>             False on failure or if kernel doesn't support ufd
> 
yes, obtained features is out parameter,
but I want to keep false uncommented and just add error_report into
syscall check, because the possible reason of failure is:
1. No syscall userfaultfd, but function expects that syscall, it reflects in
comment
2  Within syscall:  exhausted fd or out of memory (file in kernel
is allocating)
3. Problem in ioctl due to internal state of UFFD, as example
UFFDIO_API after UFFDIO_REGISTER

Also I would prefer follow migration/ram.c comment style.
> > + */
> > +static bool receive_ufd_features(uint64_t *features)
> >  {
> > -    struct uffdio_api api_struct;
> > -    uint64_t ioctl_mask;
> > +    struct uffdio_api api_struct = {0};
> > +    int ufd;
> > +    bool ret = true;
> > +
> > +    /* if we are here __NR_userfaultfd should exists */
> > +    ufd = syscall(__NR_userfaultfd, O_CLOEXEC);
> > +    if (ufd == -1) {
> > +        return false;
> > +    }
> >  
> > +    /* ask features */
> >      api_struct.api = UFFD_API;
> >      api_struct.features = 0;
> >      if (ioctl(ufd, UFFDIO_API, &api_struct)) {
> > -        error_report("%s: UFFDIO_API failed: %s", __func__
> > +        error_report("%s: UFFDIO_API failed: %s", __func__,
> >                       strerror(errno));
> > +        ret = false;
> > +        goto release_ufd;
> > +    }
> > +
> > +    *features = api_struct.features;
> > +
> > +release_ufd:
> > +    close(ufd);
> > +    return ret;
> > +}
> 
> Needs a comment; perhaps something like:
>   * Called once on a newly opened ufd, can request specific features.
>   * Returns: True on success
> 
> > +static bool request_ufd_features(int ufd, uint64_t features)
> > +{
> > +    struct uffdio_api api_struct = {0};
> > +    uint64_t ioctl_mask;
> > +
> > +    api_struct.api = UFFD_API;
> > +    api_struct.features = features;
> > +    if (ioctl(ufd, UFFDIO_API, &api_struct)) {
> > +        error_report("%s failed: UFFDIO_API failed: %s", __func__,
> > +                strerror(errno));
> >          return false;
> >      }
> >  
> > -    ioctl_mask = (__u64)1 << _UFFDIO_REGISTER |
> > -                 (__u64)1 << _UFFDIO_UNREGISTER;
> > +    ioctl_mask = 1 << _UFFDIO_REGISTER |
> > +                 1 << _UFFDIO_UNREGISTER;
> >      if ((api_struct.ioctls & ioctl_mask) != ioctl_mask) {
> >          error_report("Missing userfault features: %" PRIx64,
> >                       (uint64_t)(~api_struct.ioctls & ioctl_mask));
> >          return false;
> >      }
> >  
> > +    return true;
> > +}
> > +
> > +static bool ufd_check_and_apply(int ufd, MigrationIncomingState *mis)
> > +{
> > +    uint64_t asked_features = 0;
> > +    uint64_t supported_features;
> > +
> > +    /*
> > +     * it's not possible to
> > +     * request UFFD_API twice per one fd
> > +     */
> > +    if (!receive_ufd_features(&supported_features)) {
> > +        error_report("%s failed", __func__);
> > +        return false;
> > +    }
> > +
> > +    /*
> > +     * request features, even if asked_features is 0, due to
> > +     * kernel expects UFFD_API before UFFDIO_REGISTER, per
> > +     * userfault file descriptor
> > +     */
> > +    if (!request_ufd_features(ufd, asked_features)) {
> > +        error_report("%s failed: features %" PRIu64, __func__,
> > +                asked_features);
> > +        return false;
> > +    }
> > +
> >      if (getpagesize() != ram_pagesize_summary()) {
> >          bool have_hp = false;
> >          /* We've got a huge page */
> >  #ifdef UFFD_FEATURE_MISSING_HUGETLBFS
> > -        have_hp = api_struct.features & UFFD_FEATURE_MISSING_HUGETLBFS;
> > +        have_hp = supported_features & UFFD_FEATURE_MISSING_HUGETLBFS;
> >  #endif
> >          if (!have_hp) {
> >              error_report("Userfault on this host does not support huge pages");
> > @@ -136,7 +200,7 @@ bool postcopy_ram_supported_by_host(MigrationIncomingState *mis)
> >      }
> >  
> >      /* Version and features check */
> > -    if (!ufd_version_check(ufd, mis)) {
> > +    if (!ufd_check_and_apply(ufd, mis)) {
> >          goto out;
> >      }
> >  
> > @@ -513,7 +577,7 @@ 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, mis)) {
> > +    if (!ufd_check_and_apply(mis->userfault_fd, mis)) {
> >          return -1;
> >      }
> >  
> > -- 
> > 1.9.1
> 
> Dave
> 
> --
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> 

-- 

BR
Alexey