[Qemu-devel] Re: [PATCH 1/2] Pad iommu with an empty slot (necessary for SunOS 4.1.4)

[Blue Swirl]

On 5/8/10, Artyom Tarasenko <address@hidden> wrote:
> On the real hardware (SS-5, LX) the MMU is not padded, but aliased.
>  Software shouldn't use aliased addresses, neither should it crash
>  when it uses (on the real hardware it wouldn't). Using empty_slot
>  instead of aliasing can help with debugging such accesses.

TurboSPARC Microprocessor User's Manual shows that there are
additional pages after the main IOMMU for AFX registers. So this is
not board specific, but depends on CPU/IOMMU versions.

One approach would be that IOMMU_NREGS would be increased to cover
these registers (with the bump in savevm version field) and
iommu_init1() should check the version field to see how much MMIO to
provide.

But in order to avoid the savevm version change, iommu_init1() could
just install dummy MMIO (in the TurboSPARC case), if OBP does not care
if the read back data matches what has been written earlier. Because
from OBP point of view this is identical to what your patch results
in, I'd suppose this approach would also work.