Hi Michael, > 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.
This makes sense. LOAD_REG_ADDR() does ld reg,name@got(r2) and _GLOBAL_TOC sets r2 based on r12 and .TOC. . Looking at e.g. https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1610846.html it seems that we use GOT and TOC largely interchangeably... so assuming I haven't completely misunderstood, the change this patch makes seems to make sense to me. :) > 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. I checked and there isn't anywhere else the functions are called, so this will now cover everything. > 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. I assume that something (the module loader?) patches the callsite to restore r2 after the function call? I imagine something must otherwise things would fall apart pretty quickly... > Fixes: c1fe190c0672 ("powerpc: Add force enable of DAWR on P9 option") That patch seems to only affect the DA_W_R not the DA_B_R - how does it cause this bug? All in all this looks good to me: Reviewed-by: Daniel Axtens <d...@axtens.net> Kind regards, Daniel > Signed-off-by: Michael Ellerman <m...@ellerman.id.au> > --- > arch/powerpc/kvm/book3s_hv_rmhandlers.S | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/arch/powerpc/kvm/book3s_hv_rmhandlers.S > b/arch/powerpc/kvm/book3s_hv_rmhandlers.S > index 90484425a1e6..30a8a07cff18 100644 > --- a/arch/powerpc/kvm/book3s_hv_rmhandlers.S > +++ b/arch/powerpc/kvm/book3s_hv_rmhandlers.S > @@ -1999,7 +1999,7 @@ END_FTR_SECTION_IFSET(CPU_FTR_ARCH_300) > .globl hcall_real_table_end > hcall_real_table_end: > > -_GLOBAL(kvmppc_h_set_xdabr) > +_GLOBAL_TOC(kvmppc_h_set_xdabr) > EXPORT_SYMBOL_GPL(kvmppc_h_set_xdabr) > andi. r0, r5, DABRX_USER | DABRX_KERNEL > beq 6f > @@ -2009,7 +2009,7 @@ EXPORT_SYMBOL_GPL(kvmppc_h_set_xdabr) > 6: li r3, H_PARAMETER > blr > > -_GLOBAL(kvmppc_h_set_dabr) > +_GLOBAL_TOC(kvmppc_h_set_dabr) > EXPORT_SYMBOL_GPL(kvmppc_h_set_dabr) > li r5, DABRX_USER | DABRX_KERNEL > 3: > -- > 2.25.1
