Re: [Qemu-devel] [PATCH for-2.4 v2 1/2] vhost: add vhost_has_free_slot() interface

[Michael S. Tsirkin]

On Thu, Jul 30, 2015 at 08:16:54AM +0200, Igor Mammedov wrote:
> On Wed, 29 Jul 2015 18:11:14 +0300
> "Michael S. Tsirkin" <address@hidden> wrote:
> 
> > On Wed, Jul 29, 2015 at 01:49:48PM +0200, Igor Mammedov wrote:
> > > it will allow for other parts of QEMU check if it's safe
> > > to map memory region during hotplug/runtime.
> > > That way hotplug path will have a chance to cancel
> > > hotplug operation instead of crashing in vhost_commit().
> > > 
> > > Signed-off-by: Igor Mammedov <address@hidden>
> > > ---
> > > v2:
> > >   * replace probbing with checking for
> > >     /sys/module/vhost/parameters/max_mem_regions and
> > >     if it's missing has non wrong value return
> > >     hardcoded legacy limit (64 slots).
> > > ---
> > >  hw/virtio/vhost-backend.c         | 21 ++++++++++++++++++++-
> > >  hw/virtio/vhost-user.c            |  8 +++++++-
> > >  hw/virtio/vhost.c                 | 21 +++++++++++++++++++++
> > >  include/hw/virtio/vhost-backend.h |  2 ++
> > >  include/hw/virtio/vhost.h         |  1 +
> > >  stubs/Makefile.objs               |  1 +
> > >  stubs/vhost.c                     |  6 ++++++
> > >  7 files changed, 58 insertions(+), 2 deletions(-)
> > >  create mode 100644 stubs/vhost.c
> > > 
> > > diff --git a/hw/virtio/vhost-backend.c b/hw/virtio/vhost-backend.c
> > > index 4d68a27..11ec669 100644
> > > --- a/hw/virtio/vhost-backend.c
> > > +++ b/hw/virtio/vhost-backend.c
> > > @@ -11,6 +11,7 @@
> > >  #include "hw/virtio/vhost.h"
> > >  #include "hw/virtio/vhost-backend.h"
> > >  #include "qemu/error-report.h"
> > > +#include "linux/vhost.h"
> > >  
> > >  #include <sys/ioctl.h>
> > >  
> > > @@ -42,11 +43,29 @@ static int vhost_kernel_cleanup(struct vhost_dev *dev)
> > >      return close(fd);
> > >  }
> > >  
> > > +static int vhost_kernel_memslots_limit(struct vhost_dev *dev)
> > > +{
> > > +    int limit = 64;
> > > +    char *s;
> > > +
> > > +    if 
> > > (g_file_get_contents("/sys/module/vhost/parameters/max_mem_regions",
> > > +                            &s, NULL, NULL)) {
> > > +        uint64_t val = g_ascii_strtoull(s, NULL, 10);
> > > +        if (!((val == G_MAXUINT64 || !val) && errno)) {
> > > +            return val;
> > > +        }
> > > +        error_report("ignoring invalid max_mem_regions value in vhost 
> > > module:"
> > > +                     " %s", s);
> > > +    }
> > > +    return limit;
> > > +}
> > > +
> > >  static const VhostOps kernel_ops = {
> > >          .backend_type = VHOST_BACKEND_TYPE_KERNEL,
> > >          .vhost_call = vhost_kernel_call,
> > >          .vhost_backend_init = vhost_kernel_init,
> > > -        .vhost_backend_cleanup = vhost_kernel_cleanup
> > > +        .vhost_backend_cleanup = vhost_kernel_cleanup,
> > > +        .vhost_backend_memslots_limit = vhost_kernel_memslots_limit
> > >  };
> > >  
> > >  int vhost_set_backend_type(struct vhost_dev *dev, VhostBackendType 
> > > backend_type)
> > > diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
> > > index e7ab829..acdfd04 100644
> > > --- a/hw/virtio/vhost-user.c
> > > +++ b/hw/virtio/vhost-user.c
> > > @@ -343,9 +343,15 @@ static int vhost_user_cleanup(struct vhost_dev *dev)
> > >      return 0;
> > >  }
> > >  
> > > +static int vhost_user_memslots_limit(struct vhost_dev *dev)
> > > +{
> > > +    return VHOST_MEMORY_MAX_NREGIONS;
> > > +}
> > > +
> > >  const VhostOps user_ops = {
> > >          .backend_type = VHOST_BACKEND_TYPE_USER,
> > >          .vhost_call = vhost_user_call,
> > >          .vhost_backend_init = vhost_user_init,
> > > -        .vhost_backend_cleanup = vhost_user_cleanup
> > > +        .vhost_backend_cleanup = vhost_user_cleanup,
> > > +        .vhost_backend_memslots_limit = vhost_user_memslots_limit
> > >          };
> > > diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> > > index 2712c6f..e964004 100644
> > > --- a/hw/virtio/vhost.c
> > > +++ b/hw/virtio/vhost.c
> > > @@ -26,6 +26,18 @@
> > >  
> > >  static struct vhost_log *vhost_log;
> > >  
> > > +static int used_memslots;
> > > +static int memslots_limit = -1;
> > 
> > I suspect this need to be in some structure, refcounted by vhost devices
> > using it. vhost_kernel_memslots_limit would then be called when 1st
> > vhost device is added.
> 
> vhost_kernel_memslots_limit() should be called for every added vhost device,
> since mixed vhost backends may have different limit.
> 
> refcounting might help with disabling limit check when all vhost devices
> are hot-unplugged and user will be able to plug in more memory and won't be
> able to add vhost device with lower limit.
> but I see such usecase highly not probable, hence it's considered
> adding refcounting as over-engineered way.

I don't see what's not probable here. It's trivial to reproduce.
One reason to do this is if you want to tweak the limit.
vhost needs to be unloaded for this.


> > 
> > > +
> > > +bool vhost_has_free_slot(void)
> > > +{
> > > +    if (memslots_limit >= 0) {
> > > +        return memslots_limit > used_memslots;
> > > +    }
> > > +
> > > +    return true;
> > 
> > Pls rewrite this using logic operators.
> > Something like
> >     return memslots_limit < 0 || memslots_limit > used_memslots; ?
> sure
> 
> > 
> > > +}
> > > +
> > >  static void vhost_dev_sync_region(struct vhost_dev *dev,
> > >                                    MemoryRegionSection *section,
> > >                                    uint64_t mfirst, uint64_t mlast,
> > > @@ -457,6 +469,7 @@ static void vhost_set_memory(MemoryListener *listener,
> > >      dev->mem_changed_start_addr = MIN(dev->mem_changed_start_addr, 
> > > start_addr);
> > >      dev->mem_changed_end_addr = MAX(dev->mem_changed_end_addr, 
> > > start_addr + size - 1);
> > >      dev->memory_changed = true;
> > > +    used_memslots = dev->mem->nregions;
> > >  }
> > >  
> > >  static bool vhost_section(MemoryRegionSection *section)
> > > @@ -1119,6 +1132,14 @@ int vhost_dev_start(struct vhost_dev *hdev, 
> > > VirtIODevice *vdev)
> > >      if (r < 0) {
> > >          goto fail_features;
> > >      }
> > > +
> > > +    r = hdev->vhost_ops->vhost_backend_memslots_limit(hdev);
> > > +    if (memslots_limit > 0) {
> > > +        memslots_limit = MIN(memslots_limit, r);
> > > +    } else {
> > > +        memslots_limit = r;
> > > +    }
> > > +
> > >      r = hdev->vhost_ops->vhost_call(hdev, VHOST_SET_MEM_TABLE, 
> > > hdev->mem);
> > >      if (r < 0) {
> > >          r = -errno;
> > 
> > Doing stuff on start but not stop is almost always a mistake.
> > In this case, when all vhost devices go away, one can
> > add more memory again, right?
> yep, it's possible provided vhost_call(VHOST_SET_MEM_TABLE) succeeds,
> if it's not then device addition fails gracefully.

VHOST_SET_MEM_TABLE on which device? All fds are gone.

> > 
> > > diff --git a/include/hw/virtio/vhost-backend.h 
> > > b/include/hw/virtio/vhost-backend.h
> > > index e472f29..28b6714 100644
> > > --- a/include/hw/virtio/vhost-backend.h
> > > +++ b/include/hw/virtio/vhost-backend.h
> > > @@ -24,12 +24,14 @@ typedef int (*vhost_call)(struct vhost_dev *dev, 
> > > unsigned long int request,
> > >               void *arg);
> > >  typedef int (*vhost_backend_init)(struct vhost_dev *dev, void *opaque);
> > >  typedef int (*vhost_backend_cleanup)(struct vhost_dev *dev);
> > > +typedef int (*vhost_backend_memslots_limit)(struct vhost_dev *dev);
> > >  
> > >  typedef struct VhostOps {
> > >      VhostBackendType backend_type;
> > >      vhost_call vhost_call;
> > >      vhost_backend_init vhost_backend_init;
> > >      vhost_backend_cleanup vhost_backend_cleanup;
> > > +    vhost_backend_memslots_limit vhost_backend_memslots_limit;
> > >  } VhostOps;
> > >  
> > >  extern const VhostOps user_ops;
> > > diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h
> > > index dd51050..17ff7b6 100644
> > > --- a/include/hw/virtio/vhost.h
> > > +++ b/include/hw/virtio/vhost.h
> > > @@ -81,4 +81,5 @@ uint64_t vhost_get_features(struct vhost_dev *hdev, 
> > > const int *feature_bits,
> > >                              uint64_t features);
> > >  void vhost_ack_features(struct vhost_dev *hdev, const int *feature_bits,
> > >                          uint64_t features);
> > > +bool vhost_has_free_slot(void);
> > >  #endif
> > > diff --git a/stubs/Makefile.objs b/stubs/Makefile.objs
> > > index 9937a12..d2f1b21 100644
> > > --- a/stubs/Makefile.objs
> > > +++ b/stubs/Makefile.objs
> > > @@ -38,3 +38,4 @@ stub-obj-$(CONFIG_WIN32) += fd-register.o
> > >  stub-obj-y += cpus.o
> > >  stub-obj-y += kvm.o
> > >  stub-obj-y += qmp_pc_dimm_device_list.o
> > > +stub-obj-y += vhost.o
> > > diff --git a/stubs/vhost.c b/stubs/vhost.c
> > > new file mode 100644
> > > index 0000000..d346b85
> > > --- /dev/null
> > > +++ b/stubs/vhost.c
> > > @@ -0,0 +1,6 @@
> > > +#include "hw/virtio/vhost.h"
> > > +
> > > +bool vhost_has_free_slot(void)
> > > +{
> > > +    return true;
> > > +}
> > > -- 
> > > 1.8.3.1