KVM: PPC: Book3S HV: Use GLOBAL_TOC for kvmppc_h_set_dabr/xdabr()

KVM: PPC: Book3S HV: Use GLOBAL_TOC for kvmppc_h_set_dabr/xdabr()

kvmppc_h_set_dabr(), and kvmppc_h_set_xdabr() which jumps into
it, need to use _GLOBAL_TOC to setup the kernel TOC pointer, because
kvmppc_h_set_dabr() uses LOAD_REG_ADDR() to load dawr_force_enable.

When called from hcall_try_real_mode() we have the kernel TOC in r2,
established near the start of kvmppc_interrupt_hv(), so there is no
issue.

But they can also be called from kvmppc_pseries_do_hcall() which is
module code, so the access ends up happening with the kvm-hv module's
r2, which will not point at dawr_force_enable and could even cause a
fault.

With the current code layout and compilers we haven't observed a fault
in practice, the load hits somewhere in kvm-hv.ko and silently returns
some bogus value.

Note that we we expect p8/p9 guests to use the DAWR, but SLOF uses
h_set_dabr() to test if sc1 works correctly, see SLOF's
lib/libhvcall/brokensc1.c.

Fixes: c1fe190c0672 ("powerpc: Add force enable of DAWR on P9 option")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Daniel Axtens <dja@axtens.net>
Link: https://lore.kernel.org/r/20210923151031.72408-1-mpe@ellerman.id.au