On Wed, 17 Mar 2021 at 20:17, Dr. David Alan Gilbert <dgilb...@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.
If you want to use low bits as flags for other stuff, you should have a compile time assert that you have the number of bits you expect, or otherwise force a compile error. Otherwise you'll end up with unpleasant surprises like this one... I think that for the cpu-all.h uses of low bits we would end up with a compile error for excessively small TARGET_PAGE_BITS because we define the bits like this: #define TLB_DISCARD_WRITE (1 << (TARGET_PAGE_BITS_MIN - 6)) and I expect the compiler will complain if the RHS of the '<<' is a negative constant. But I don't know if that's deliberate or a happy accident :-) thanks -- PMM
