Re: [Qemu-ppc] [Qemu-devel] [PATCH 3/3] target/ppc: filter out non-zero PCR values when using TCG

[David Gibson]

On Fri, Jun 15, 2018 at 12:00:20AM +0200, Greg Kurz wrote:
> On Thu, 14 Jun 2018 09:52:55 -1000
> Richard Henderson <address@hidden> wrote:
> 
> > On 06/12/2018 10:19 PM, Greg Kurz wrote:
> > > I'm confused... I don't see anything related to HTM in TCG. Also we have
> > > the following in cap_htm_apply():
> > > 
> > >     if (tcg_enabled()) {
> > >         error_setg(errp,
> > >                    "No Transactional Memory support in TCG, try 
> > > cap-htm=off");
> > > 
> > > I'm probably missing something... can you enlighten me ?  
> > 
> > One of the two IBM machines -- and I thought it was ppc not s390x, but the 
> > code
> > you quote seems to deny that memory -- has stub support for TM within TCG.
> 
> Oh ? I didn't know and didn't check :)

That is true, there are stub implementations of the TM instructions.

> 
> > To wit, the instructions are recognized and transactions always fail.  
> > Which is
> > not a bad way to test the required fallback paths that rarely fail on 
> > hardware.
> >  ;-)
> 
> If TM instructions don't cause an exception, I guess its reasonable to say
> they're supported :)

That argument has come up before, and I disagree.  I don't think it's
reasonable to advertise TM support as available if the instructions
always fail.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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