Now that I'm making the branch microop always use a fixed absolute
micropc, the only place I wasn't already using it, the CPUID
instruction, needs to change. The problem is, as things are implemented,
it really has to be able to compute it's target. The CPUID instruction
basically queries a mostly but not completely static pool of information
about the CPU it's run on. For instance, it can tell you the size of
various caches, what version the CPU is, who the manufacturer was, what
instruction extensions are supported (that's partially where the info in
/proc/cpuinfo comes from), blah blah blah. It's not completely static
for two reasons. First, sometimes certain extensions are implemented in
only some modes. I believe some CPUs turn off the bits of instructions
that won't work in the current mode, although I'm not sure of that and I
think it's done inconsistently among processors. Second, I believe Intel
now allows you to tamper with the values returned by CPUID in order to
allow a virtualized guest to query freely and not see capabilities that
wouldn't work or that it shouldn't use.
Right now, my implementation of CPUID does a little munging on the
"function" code, which specifies what information you want, and then
goes into what is essentially a big case statement/computed branch that
puts the right values in the right registers and then returns. As I
mentioned, since I'll no longer be able to do computed branches, this
will no longer work. There are lots of other limitations too like having
lots of microops to function as a basic lookup table, and the fact that
the information is static and completely unconfigurable. For instance,
the cache would always reported as the same size, and if some benchmark
tried to use that value to behave in a certain way, it wouldn't do what
it was supposed to. What I'm thinking I'd want to do is one of two
things. Either the CPUID instruction should do a series of loads out of
an actual lookup ROM/RAM somewhere outside of the CPU, or there could be
a CPUID device which would allow it to respond in intelligent ways
depending on the CPU mode, for instance. I'm favoring sticking a ROM in
the memory system somewhere. Also, I'd like to put in some sort of
configuration interface that would allow the configs to program in what
CPUID should say if it needs to reflect the actual hardware or someone
wants to add a new function, for instance.
What do you guys think?
Gabe
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