Re: [PATCH v5 3/6] migration: Maintain postcopy faulted addresses

[Peter Xu]

On Wed, Oct 21, 2020 at 06:42:19PM +0100, Dr. David Alan Gilbert wrote:
> * Peter Xu (peterx@redhat.com) wrote:
> > On Wed, Oct 21, 2020 at 03:23:45PM +0100, Dr. David Alan Gilbert wrote:
> > > > @@ -354,8 +368,33 @@ int 
> > > > migrate_send_rp_message_req_pages(MigrationIncomingState *mis,
> > > >  }
> > > >  
> > > >  int migrate_send_rp_req_pages(MigrationIncomingState *mis,
> > > > -                              RAMBlock *rb, ram_addr_t start)
> > > > +                              RAMBlock *rb, ram_addr_t start, uint64_t 
> > > > haddr)
> > > >  {
> > > > +    void *aligned = (void *)(uintptr_t)(haddr & 
> > > > qemu_real_host_page_mask);
> > > 
> > > Can you remind me, what happens here for hugepages?
> > 
> > Sure.  Previously it was:
> > 
> >   (haddr & (-qemu_target_page_size())
> > 
> > Now it is:
> > 
> >   (haddr & qemu_real_host_page_mask)
> > 
> > Basically we changed the psize alignment from guest to host.
> > 
> > The bug triggered previously on ppc64 where host_psize=64k, then when guest
> > psize is smaller, e.g., 4k, we can have some addr that aligned to 4k rather
> > than 64k, so we failed later on checking the host psize alignment (because 
> > this
> > pointer should point to a host page, so it should align with host psize).
> 
> But my question is what happens when we have say a 2MB hugepage?

Oops, I definitely misread.

Good point, I think it can break hugepages.  So the mask should really be
"(qemu_ram_pagesize(rb) - 1)".

I'll fix and smoke it with some huge pages before another repost.

Thanks!

-- 
Peter Xu