Re: [PATCH 11/26] KVM: PPC: Make RMO a define

[Alexander Graf <agraf@suse.de>]

On 29.06.2010, at 09:32, Segher Boessenkool wrote:

>>>> On PowerPC it's very normal to not support all of the physical RAM =
in real mode.
>>>=20
>>> Oh?  Are you referring to "real mode limit", or 32-bit =
implementations with
>>> more than 32 address lines, or something else?
>>=20
>> The former.
>=20
> Okay.  In that case, the hypervisor can usually access all of physical
> ram directly, unless it limits itself; and a supervisor gets either
> no real mode access, or all ram, or some part -- whatever the =
hypervisor
> likes.
>=20
> If you access outside of ram, you will most likely get a machine =
check.
> Depends on the implementation (like most things here).
>=20
>>> Either way, RMO is a really bad name for this, since that name is =
already
>>> used for a similar but different concept.
>>=20
>> It's the same concept, no? Not all physical memory is accessible from =
real mode.
>=20
> I think you're looking for "real mode limit":
>=20
> The concept is called "offset real mode".
>=20
> RMO ("real mode offset") is the value that is ORed to an effective =
address
> to make the physical address (not added, "offset" is a pretty bad =
name).
> RML ("real mode limit") is the value that has to be bigger than the =
effective
> address or you will get an exception.
> RMA ("real mode area") is the name for the range of addresses usable =
in offset
> real mode.
>=20
>>> Also, it seems you construct the physical address by masking out =
bits from
>>> the effective address.  Most implementations will trap or machine =
check if
>>> you address outside of physical address space, instead.
>>=20
>> Well the only case where I remember to have hit a real RMO case is on =
the PS3 - that issues a data/instruction storage interrupt when =
accessing anything > 8MB in real mode.
>>=20
>> So I'd argue this is heavily implementation specific.
>=20
> It is.  So what is the behaviour you want to implement?

The one below.

>=20
>> Apart from that what I'm trying to cover is that on ppc64 accessing =
0xc0000000000000 in real mode gets you 0x0. Is there a better name for =
this?
>=20
> (You missed two zeroes).
> In hypervisor real mode, the top few bits are magic.  They are used =
for e.g.
> enabling hypervisor offset real mode.
> In supervisor real mode, those bits are ignored (and all other bits =
that do
> not correspond to physical address lines may also be ignored).

So which bits exactly are reserved? I couldn't find a reference to that =
part.

> Maybe you want to call it physical_address_mask or similar?

PAM - doesn't sound bad :).


Alex