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 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.

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