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

Jordan Niethe jniethe5 at gmail.com
Thu Sep 30 16:59:55 AEST 2021


On Thu, Sep 30, 2021 at 4:13 PM Daniel Axtens <dja at axtens.net> wrote:
>
> 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 at 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?

Isn't it that patch which adds the LOAD_REG_ADDR(r11,
dawr_force_enable) to kvmppc_h_set_dabr() which is the problem?

>
> All in all this looks good to me:
> Reviewed-by: Daniel Axtens <dja at axtens.net>
>
> Kind regards,
> Daniel
>
> > Signed-off-by: Michael Ellerman <mpe at 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


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