Re: [Qemu-devel] [PATCH 2/2] target-ppc: fix xscmpodp and xscmpudp decoding

[Tom Musta]

The modern versions of the ISA require that reserved instruction bits be ignored for server class processors (see Book I, section 1.3.3).  I believe older versions of the ISA allowed for Illegal Instruction Interrupt or "Boundedly Undefined", which is, of course, much less specific.  QEMU supports implementations from both eras and, as a general rule, flags this situation as an illegal instruction.

So I would expect that real hardware will ignore the bit.  You will still be left with the choice of making this decoder consistent with the hardware or consistent with the rest of QEMU :)   When I added support for VSX, the intent was the latter.

On Tue, Sep 22, 2015 at 4:28 PM, Aurelien Jarno <address@hidden> wrote:

On 2015-09-22 12:26, Thomas Huth wrote:
> On 13/09/15 23:03, Aurelien Jarno wrote:
> > The xscmpodp and xscmpudp instructions only have the AX, BX bits in
> > there encoding, the lowest bit (usually TX) is marked as an invalid
> > bit. We therefore can't decode them with GEN_XX2FORM, which decodes
> > the two lowest bit.
> >
> > Introduce a new form GEN_XX2FORM, which decodes AX and BX and mark
> > the lowest bit as invalid.
> >
> > Cc: Tom Musta <address@hidden>
> > Cc: Alexander Graf <address@hidden>
> > Cc: address@hidden
> > Signed-off-by: Aurelien Jarno <address@hidden>
> > ---
> >  target-ppc/translate.c | 11 +++++++++--
> >  1 file changed, 9 insertions(+), 2 deletions(-)
> >
> > diff --git a/target-ppc/translate.c b/target-ppc/translate.c
> > index 84c5cea..c0eed13 100644
> > --- a/target-ppc/translate.c
> > +++ b/target-ppc/translate.c
> > @@ -10670,6 +10670,13 @@ GEN_HANDLER2_E(name, #name, 0x3C, opc2 | 1, opc3, 0, PPC_NONE, fl2), \
> >  GEN_HANDLER2_E(name, #name, 0x3C, opc2 | 2, opc3, 0, PPC_NONE, fl2), \
> >  GEN_HANDLER2_E(name, #name, 0x3C, opc2 | 3, opc3, 0, PPC_NONE, fl2)
> >
> > +#undef GEN_XX2IFORM
> > +#define GEN_XX2IFORM(name, opc2, opc3, fl2)                           \
> > +GEN_HANDLER2_E(name, #name, 0x3C, opc2 | 0, opc3, 1, PPC_NONE, fl2), \
> > +GEN_HANDLER2_E(name, #name, 0x3C, opc2 | 1, opc3, 1, PPC_NONE, fl2), \
> > +GEN_HANDLER2_E(name, #name, 0x3C, opc2 | 2, opc3, 1, PPC_NONE, fl2), \
> > +GEN_HANDLER2_E(name, #name, 0x3C, opc2 | 3, opc3, 1, PPC_NONE, fl2)
> > +
> >  #undef GEN_XX3_RC_FORM
> >  #define GEN_XX3_RC_FORM(name, opc2, opc3, fl2)                          \
> >  GEN_HANDLER2_E(name, #name, 0x3C, opc2 | 0x00, opc3 | 0x00, 0, PPC_NONE, fl2), \
> > @@ -10731,8 +10738,8 @@ GEN_XX3FORM(xsnmaddadp, 0x04, 0x14, PPC2_VSX),
> >  GEN_XX3FORM(xsnmaddmdp, 0x04, 0x15, PPC2_VSX),
> >  GEN_XX3FORM(xsnmsubadp, 0x04, 0x16, PPC2_VSX),
> >  GEN_XX3FORM(xsnmsubmdp, 0x04, 0x17, PPC2_VSX),
> > -GEN_XX2FORM(xscmpodp,  0x0C, 0x05, PPC2_VSX),
> > -GEN_XX2FORM(xscmpudp,  0x0C, 0x04, PPC2_VSX),
> > +GEN_XX2IFORM(xscmpodp,  0x0C, 0x05, PPC2_VSX),
> > +GEN_XX2IFORM(xscmpudp,  0x0C, 0x04, PPC2_VSX),
>
> According to PowerISA 2.07, xscmpodp and xscmpudp are of type XX3, not
> of type XX2 ... so should this macro maybe rather be named XX3IFORM instead?

Indeed, I have chosen the name without actually realizing the manual
also give names. Then I do wonder if the lower bit is really decoded as
invalid, I wouldn't be surprised it is actually just ignored.

I'll try to do some tests on real hardware and come up with a fixup
patch.

Aurelien

--
Aurelien Jarno                          GPG: 4096R/1DDD8C9B
address@hidden                 http://www.aurel32.net