Re: [Qemu-devel] [PATCH] vl: Delay initialization of memory backends

[Eduardo Habkost]

On Thu, Sep 01, 2016 at 02:39:37PM -0300, Eduardo Habkost wrote:
> On Thu, Sep 01, 2016 at 05:41:52PM +0200, Paolo Bonzini wrote:
> > On 01/09/2016 17:10, Eduardo Habkost wrote:
> > > Ouch. It looks like the ordering requirements are messier than I
> > > thought. vhost-user depends on the memory backends to be already
> > > initialized.
> > 
> > You could also look at delaying initialization of vhost-user, not
> > sending anything on the wire until after machine creation.
> 
> I was wishing the bug could be fixed without the need to touch
> vhost, but I will take a look.
> 
> BTW, the vhost error is actually happening inside a VCPU thread,
> after everything was supposed to be fully initialized.  Maybe the
> memory listener logic in vhost.c is broken somehow?

This is getting hairier.

Summary:

1) vhost_user_set_mem_table() fails because dev->mem->nregions is 0
2) dev->mem->nregions is supposed to get new entries based on the
   memory listener callbacks
3) vhost_region_add() gets called properly, and calls
   vhost_set_memory(), but:
4) vhost_set_memory() forces add=false if
   memory_region_get_dirty_log_mask(section->mr) & ~(1 << 
DIRTY_MEMORY_MIGRATION)
   (I have no idea why)
5) memory_region_init_ram_from_file() sets:
   mr->dirty_log_mask = tcg_enabled() ? (1 << DIRTY_MEMORY_CODE) : 0;
   (I don't understand what are the consequences of this)
6) The tcg_enabled() check above is broken if the memory region
   is created before configure_accelerator() is called. My patch
   moves memory backend initialization after
   configure_accelerator()

I'm very confused. My patch seems to fix the dirty_log_mask
initialization at (5) by accident? But for some reason this
breaks vhost-user and makes it ignore all memory regions if using
TCG? (vhost-user-test forces accel=tcg, BTW)

As I have no idea why vhost_set_memory() ignores the memory
region based on memory_region_get_dirty_log_mask(), I don't know
what's supposed to be happening here. Any help would be
appreciated.

-- 
Eduardo