Re: of AVR target page size

[Dr. David Alan Gilbert]

* 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