On 3/17/22 11:33, Peter Maydell wrote:
The behaviour of r0 in the shadow register sets is definitely
underspecified, but I really don't believe that r0 is a normal
writeable register for everything except the crs=0 set, which
is what you've implemented here. My best guess is:
* registers are implemented as a pile of RAM, including r0
* on reset the set-0 r0 is reset to 0, but nothing else is
(this bit's actually in the spec)
* writes to r0 are always discarded, except for the special
case of wrprs
Thanks for the insight. It certainly sounds plausible.
I'm tempted to suggest we should make our tbflags bit
"we know r0 is zero" -- the guest doesn't have many ways
to switch register set, basically I think just eret and taking
an external interrupt, and those either happen outside the
TB or are going to end the TB anyway. Can we make
cpu_get_tb_cpu_state() simply set the TB flag if
env->shadow_regs[crs][0] == 0
or have I missed something that means that won't work?
Yes, this is easy.
(I actually wouldn't care to bet much money on wrprs being
unable to write to register-set-0 r0. It would be interesting
to test that on the real hardware.)
Indeed. I'm tempted to treat them all the same.
r~
