Re: [PATCH] target/ppc: Fix sc instruction handling of LEV field

[Nicholas Piggin]

On Thu Jun 22, 2023 at 6:00 PM AEST, Harsh Prateek Bora wrote:
>
>
> On 6/21/23 16:39, Nicholas Piggin wrote:
> > The top bits of the LEV field of the sc instruction are to be treated as
> > as a reserved field rather than a reserved value, meaning LEV is
> > effectively the bottom bit. LEV=0xF should be treated as LEV=1 and be
> > a hypercall, for example.
> > 
> > This changes the instruction execution to just set lev from the low bit
> > of the field. Processors which don't support the LEV field will continue
> > to ignore it.
> > 
> > ISA v3.1 defines LEV to be 2 bits, in order to add the 'sc 2' ultracall
> > instruction. TCG does not support Ultravisor, so don't worry about
> > that bit.
> > 
> > Suggested-by: "Harsh Prateek Bora" <harshpb@linux.ibm.com>
> > Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> > ---
> > This should probably go ahead of the ISA 3.1 LEV in SRR1 patch. I
> > don't think they need to be backported to stable though, have not
> > caused any real problems.
> > 
> > Thanks to Harsh for spotting it.
> > 
> > Thanks,
> > Nick
> > 
> >   target/ppc/translate.c | 7 ++++++-
> >   1 file changed, 6 insertions(+), 1 deletion(-)
> > 
> > diff --git a/target/ppc/translate.c b/target/ppc/translate.c
> > index 15a00bd4fa..3c62f9188a 100644
> > --- a/target/ppc/translate.c
> > +++ b/target/ppc/translate.c
> > @@ -4424,7 +4424,12 @@ static void gen_sc(DisasContext *ctx)
> >   {
> >       uint32_t lev;
> >   
> > -    lev = (ctx->opcode >> 5) & 0x7F;
> > +    /*
> > +     * LEV is a 7-bit field, but the top 6 bits are treated as a reserved
> > +     * field (i.e., ignored). ISA v3.1 changes that to 5 bits, but that is
> > +     * for Ultravisor which TCG does not support, so just ignore the top 6.
> > +     */
> > +    lev = (ctx->opcode >> 5) & 0x1;
>
> should this change be applied to gen_scv() defined next to it as well ?

No. scv uses all LEV bits.

Thanks,
Nick

>
> Otherwise,
> Reviewed-by: Harsh Prateek Bora <harshpb@linux.ibm.com>
>
> >       gen_exception_err(ctx, POWERPC_SYSCALL, lev);
> >   }
> >