Re: [Qemu-devel] [PATCH v4 1/6] vhost: Move log_dirty check

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Igor Mammedov (imammedo@redhat.com) wrote:
> On Thu, 14 Dec 2017 15:20:10 +0000
> "Dr. David Alan Gilbert" <dgilbert@redhat.com> wrote:
> 
> > * Igor Mammedov (imammedo@redhat.com) wrote:
> > > On Wed, 13 Dec 2017 18:08:02 +0000
> > > "Dr. David Alan Gilbert (git)" <dgilbert@redhat.com> wrote:
> > > 
> > > > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> > > > 
> > > > Move the log_dirty check into vhost_section.
> > > > 
> > > > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > > > ---
> > > >  hw/virtio/trace-events |  3 +++
> > > >  hw/virtio/vhost.c      | 20 +++++++++++++-------
> > > >  2 files changed, 16 insertions(+), 7 deletions(-)
> > > > 
> > > > diff --git a/hw/virtio/trace-events b/hw/virtio/trace-events
> > > > index 775461ae98..4a493bcd46 100644
> > > > --- a/hw/virtio/trace-events
> > > > +++ b/hw/virtio/trace-events
> > > > @@ -1,5 +1,8 @@
> > > >  # See docs/devel/tracing.txt for syntax documentation.
> > > >  
> > > > +# hw/virtio/vhost.c
> > > > +vhost_section(const char *name, int r) "%s:%d"
> > > > +
> > > >  # hw/virtio/virtio.c
> > > >  virtqueue_alloc_element(void *elem, size_t sz, unsigned in_num, unsigned out_num) "elem %p size %zd in_num %u out_num %u"
> > > >  virtqueue_fill(void *vq, const void *elem, unsigned int len, unsigned int idx) "vq %p elem %p len %u idx %u"
> > > > diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> > > > index e4290ce93d..e923219e63 100644
> > > > --- a/hw/virtio/vhost.c
> > > > +++ b/hw/virtio/vhost.c
> > > > @@ -27,6 +27,7 @@
> > > >  #include "hw/virtio/virtio-access.h"
> > > >  #include "migration/blocker.h"
> > > >  #include "sysemu/dma.h"
> > > > +#include "trace.h"
> > > >  
> > > >  /* enabled until disconnected backend stabilizes */
> > > >  #define _VHOST_DEBUG 1
> > > > @@ -567,18 +568,12 @@ static void vhost_set_memory(MemoryListener *listener,
> > > >                                           memory_listener);
> > > >      hwaddr start_addr = section->offset_within_address_space;
> > > >      ram_addr_t size = int128_get64(section->size);
> > > > -    bool log_dirty =
> > > > -        memory_region_get_dirty_log_mask(section->mr) & ~(1 << DIRTY_MEMORY_MIGRATION);
> > > >      int s = offsetof(struct vhost_memory, regions) +
> > > >          (dev->mem->nregions + 1) * sizeof dev->mem->regions[0];
> > > >      void *ram;
> > > >  
> > > >      dev->mem = g_realloc(dev->mem, s);
> > > >  
> > > > -    if (log_dirty) {
> > > > -        add = false;
> > > > -    }
> > > > -
> > > >      assert(size);
> > > >  
> > > >      /* Optimize no-change case. At least cirrus_vga does this a lot at this time. */
> > > > @@ -611,8 +606,19 @@ static void vhost_set_memory(MemoryListener *listener,
> > > >  
> > > >  static bool vhost_section(MemoryRegionSection *section)
> > > >  {
> > > > -    return memory_region_is_ram(section->mr) &&
> > > > +    bool result;
> > > > +    bool log_dirty = memory_region_get_dirty_log_mask(section->mr) &
> > > > +                     ~(1 << DIRTY_MEMORY_MIGRATION);
> > > > +    result = memory_region_is_ram(section->mr) &&
> > > >          !memory_region_is_rom(section->mr);
> > > > +
> > > > +    /* Vhost doesn't handle any block which is doing dirty-tracking other
> > > > +     * than migration; this typically fires on VGA areas.
> > > > +     */
> > > > +    result &= !log_dirty;
> > > before patch even if log_dirty, vhost_set_memory will still proceed
> > > and may remove dirty section from memmap and set memory_changed = true
> > > 
> > > but with this patch it will just ignore such section,
> > > I'm not sure it's right (it might be right with new approach add/nop
> > > but then this patch should go after new code in place and old one
> > > if gone).
> > 
> > I thought about that, but then I came to the conclusion that the whole
> > idea is that we're supposed to be ignoring these regions - so why
> > should they cause any change to the behaviour of vhost at all?
> > Thus this way seems safer.
> the thing is that with originally with old code backend will use memap
> with not yet dirty region, then after region becomes dirty current code
> will REMOVE dirty region from map and NOTIFY backend.
> 
> While this patch is fine for new approach as memmap is build from scratch,
> it doesn't look correct for current code since region that just became
> dirty will not be removed from memap and might be used backend.
> Whether it would really break something or not I'm not sure,
> maybe Michael may Ack this patch ii using dirty region is fine.

That's OK; I can add the check to vhost_region_add_section when I
initially create it, and then swing this change around nearer to the
end of the patchset, so that now we're using the new code.

Dave

> > 
> > Dave
> > 
> > > > +
> > > > +    trace_vhost_section(section->mr->name, result);
> > > > +    return result;
> > > >  }
> > > >  
> > > >  static void vhost_begin(MemoryListener *listener)
> > > 
> > --
> > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK