Re: [Qemu-devel] [PATCH v2 01/15] configure: add the disable-tcg option

["Daniel P. Berrange" <berrange@redhat.com>]

On Mon, Jul 03, 2017 at 12:25:36PM +0200, Paolo Bonzini wrote:
> 
> 
> On 03/07/2017 12:20, Daniel P. Berrange wrote:
> >>  
> >> +if test "$tcg" = "no"; then
> >> +   for target in $target_list; do
> >> +      if test "$softmmu" = "yes"; then
> >> +        case $target in
> >> +           i386-softmmu|x86_64-softmmu)
> >> +           ;;
> >> +        *)
> >> +           error_exit "The current $target can't support disable-tcg,"\
> >> +              "only i386-softmmu|x86_64-softmmu support disable-tcg"
> >> +           ;;
> > This looks too simplistic in its logic.
> > 
> > You can disable TCG, if-and-only-if the system emulator supports KVM.
> > 
> > KVM is supported on many architectures, not only x86-64 & i386.
> > 
> > KVM is only supported if the guest emulator architecture matches the
> > host build target architecture.
> > 
> > ie if you are building an x86_64 system emulator on a PPC64 host,
> > then you can't disable TCG.
> > 
> > So this needs rewriting to *not* special case x86_64 / i386. Instead
> > you need to compare & match build target / system emulator architectures,
> > across all architectures supporting KVM.
> 
> i386-softmmu and x86_64-softmmu are singled out here, because they're
> the only targets where --disable-tcg compiles.  For the others, more
> work is needed (see patches 6-15 in Yang Zhong's series).

Even with that, you still can't disable TCG if building on a non-x86
host, since that'd leave you with no available CPU at all. So the
code still needs refactoring to check architectures properly.

> The part that is missing in this patch is disabling non-hypervisor
> targets when --disable-tcg is specified.  My original patch built only
> i386-softmmu and x86_64-softmmu if you specified --disable-tcg, see
> 
> https://lists.nongnu.org/archive/html/qemu-devel/2012-09/msg02570.html
> https://lists.nongnu.org/archive/html/qemu-devel/2012-09/msg02571.html

Regards,
Daniel
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