Re: of AVR target page size

[Dr. David Alan Gilbert]

* Michael Rolnik (mrolnik@gmail.com) wrote:
> ok. I will try to fix it.

Thanks, if you can that would be great.

Dave

> Regards,
> Michael Rolnik
> 
> On Thu, Mar 18, 2021 at 11:55 AM Dr. David Alan Gilbert <dgilbert@redhat.com>
> wrote:
> 
> > * Michael Rolnik (mrolnik@gmail.com) wrote:
> > > Hi Dave.
> > >
> > > What is the smallest supported page size?
> >
> > Currently 512 I think; in migration/ram.c we have:
> >
> > #define RAM_SAVE_FLAG_FULL     0x01 /* Obsolete, not used anymore */
> > #define RAM_SAVE_FLAG_ZERO     0x02
> > #define RAM_SAVE_FLAG_MEM_SIZE 0x04
> > #define RAM_SAVE_FLAG_PAGE     0x08
> > #define RAM_SAVE_FLAG_EOS      0x10
> > #define RAM_SAVE_FLAG_CONTINUE 0x20
> > #define RAM_SAVE_FLAG_XBZRLE   0x40
> > /* 0x80 is reserved in migration.h start with 0x100 next */
> > #define RAM_SAVE_FLAG_COMPRESS_PAGE    0x100
> >
> > so we're already using the 0x100 (256) flag.
> >
> > I spotted this yesterday because a patch tried to use the 0x200 flag.
> >
> > Dave
> > >
> > > On Wed, Mar 17, 2021 at 10:14 PM Dr. David Alan Gilbert <
> > dgilbert@redhat.com>
> > > wrote:
> > >
> > > > Hi Michael,
> > > >   I noticed your AVR code defines:
> > > >
> > > >   #define TARGET_PAGE_BITS 8
> > > >
> > > > and has an explanation of why.
> > > >
> > > > Note however that's not going to work with the current live
> > > > migration/snapshotting code, since you're a couple of bits smaller
> > > > than the smallest page size we had so far, and for many years
> > > > the RAM migration code has stolen the bottom few bits of the address
> > > > as a flag field, and has already used 0x100 up; see migration/ram.c
> > > > RAM_SAVE_FLAG_*    - and it's actually tricky to change it, because if
> > > > you change it then it'll break migration compatibility with existing
> > > > qemu's.
> > > >
> > > > Hmm.
> > > >
> > > > Dave
> > > >
> > > > --
> > > > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> > > >
> > > >
> > >
> > > --
> > > Best Regards,
> > > Michael Rolnik
> > --
> > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> >
> >
> 
> -- 
> Best Regards,
> Michael Rolnik
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK