Re: [Qemu-devel] [PATCH v3 8/9] Use qemu_put_buffer_no_copy for guest memory pages

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Mar 21, 2013 at 08:08:58PM +0200, Orit Wasserman wrote:
> On 03/21/2013 07:37 PM, Juan Quintela wrote:
> > Orit Wasserman <owasserm@redhat.com> wrote:
> >> This will remove an unneeded copy of guest memory pages.
> >> For the page header and device state we still copy the data to the
> >> static buffer the other option is to allocate the memory on demand
> >> which is more expensive.
> >>
> >> Signed-off-by: Orit Wasserman <owasserm@redhat.com>
> >> ---
> >>  arch_init.c | 2 +-
> >>  savevm.c    | 2 +-
> >>  2 files changed, 2 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/arch_init.c b/arch_init.c
> >> index 98e2bc6..27b53eb 100644
> >> --- a/arch_init.c
> >> +++ b/arch_init.c
> >> @@ -481,7 +481,7 @@ static int ram_save_block(QEMUFile *f, bool last_stage)
> >>              /* XBZRLE overflow or normal page */
> >>              if (bytes_sent == -1) {
> >>                  bytes_sent = save_block_hdr(f, block, offset, cont, RAM_SAVE_FLAG_PAGE);
> >> -                qemu_put_buffer(f, p, TARGET_PAGE_SIZE);
> >> +                qemu_put_buffer_no_copy(f, p, TARGET_PAGE_SIZE);
> >>                  bytes_sent += TARGET_PAGE_SIZE;
> >>                  acct_info.norm_pages++;
> >>              }
> > 
> > Once here, shouldn't we also change:
> > 
> > block-migration.c::blk_send()
> > 
> >     qemu_put_buffer(f, blk->buf, BLOCK_SIZE);
> > 
> > to nocopy?
> Sadly no, I looked at the code and I saw the buffer is freed immediately
> after call blk_send :(. look at mig_save_device_dirty.
> 
> Orit

Hmm if _nocopy also implies it is not safe to free the
buffer immediately, then I'd rather put this in a name
e.g. _async instead of _nocopy.

> > 
> > Again, this can be an additional patch.
> > 
> > 
> >> diff --git a/savevm.c b/savevm.c
> >> index fbfb9e3..50e8fb2 100644
> >> --- a/savevm.c
> >> +++ b/savevm.c
> >> @@ -634,7 +634,7 @@ void qemu_put_buffer_no_copy(QEMUFile *f, const uint8_t *buf, int size)
> >>          abort();
> >>      }
> >>  
> >> -    f->iov[f->iovcnt].iov_base = f->buf + f->buf_index;
> >> +    f->iov[f->iovcnt].iov_base = (uint8_t *)buf;
> >>      f->iov[f->iovcnt++].iov_len = size;
> > 
> > This bit should be in the previous patch, the error that I pointed?
> > 
> > Later, Juan.
> >