Re: target/arm: "define_arm_cp_regs" and similar for KVM

[Peter Maydell]

On Fri, 26 Feb 2021 at 17:27, Claudio Fontana <cfontana@suse.de> wrote:
>
> Hi Peter,
>
> I am trying to find out how to split properly KVM and TCG in target/arm, 
> among other things.
>
> I skipped or stubbed all define_arm_cp_regs and similar functions,
>
> and made a cpregs module that is TCG-only.
>
> Thought it is fine, as we have a kvm_arm_init_cpreg_list that throws away 
> everything TCG-related anyway later:
>
> target/arm/kvm.c:
>
> /* Initialize the ARMCPU cpreg list according to the kernel's
>  * definition of what CPU registers it knows about (and throw away
>  * the previous TCG-created cpreg list).
>  */
> int kvm_arm_init_cpreg_list(ARMCPU *cpu)

It throws away the cpreg *list*, but not the cpreg hash table.
That is, we trust KVM for "what sysregs exist and need to be migrated"
but we still
use the hashtable to find out what QEMU thinks about them and where it stores
them in the CPU state structure if it knows about them. So you still
need to keep the
define_arm_cp_regs stuff to register sysregs. Not doing so will break
any bits of the
code which rely on being able to do "synchronize kernel's idea of
sysreg state to
QEMU" and then "read the register value out of the CPU state struct". It'll also
break visibility of sysregs in the gdbstub.

thanks
-- PMM