Re: [Qemu-devel] [PATCH 1/1] target/ppc: Improve accuracy of guest HTM availability on P8s

[Sam Bobroff <sam.bobroff@au1.ibm.com>]

On Wed, Mar 29, 2017 at 07:39:25AM +0200, Thomas Huth wrote:
> On 29.03.2017 07:01, Sam Bobroff wrote:
> > On Power8 hosts it is currently theoretically possible for QEMU/KVM-HV guests
> > to receive a ibm,pa-features property indicating that HTM support is available
> > when it is not.  The situation would occur if the platform firmware of
> > a Power8 host cleared the HTM bit of the ibm,pa-features property.
> 
> Out of curiosity: Is there a machine out there where this happens?

Not that I know of... just the one who's firmware I broke on purpose for
testing ;-)

> > QEMU would query KVM for the availability of HTM, which will return no
> > support, but workaround code in kvm_arch_init_vcpu() would then
> > re-enable it because KVM_HV is in use and the processor is P8.
> > 
> > This patch adjusts the workaround in kvm_arch_init_vcpu() so that it does not
> > enable HTM (in the above case) unless the host kernel indicates to the QEMU
> > process, via the auxiliary vector, that userspace can use HTM (via the HWCAP2
> > bit KVM_FEATURE2_HTM).
> > 
> > The reason to use the value from the auxiliary vector is that it is
> > set based only on what the host kernel found in the ibm,pa-features
> > HTM bit at boot time.
> > 
> > Signed-off-by: Sam Bobroff <sam.bobroff@au1.ibm.com>
> > ---
> >  target/ppc/kvm.c | 8 ++++++--
> >  1 file changed, 6 insertions(+), 2 deletions(-)
> > 
> > diff --git a/target/ppc/kvm.c b/target/ppc/kvm.c
> > index 9f1f132cef..8a54709ae4 100644
> > --- a/target/ppc/kvm.c
> > +++ b/target/ppc/kvm.c
> > @@ -49,6 +49,7 @@
> >  #if defined(TARGET_PPC64)
> >  #include "hw/ppc/spapr_cpu_core.h"
> >  #endif
> > +#include "elf.h"
> >  
> >  //#define DEBUG_KVM
> >  
> > @@ -509,8 +510,11 @@ int kvm_arch_init_vcpu(CPUState *cs)
> >      case POWERPC_MMU_2_07:
> >          if (!cap_htm && !kvmppc_is_pr(cs->kvm_state)) {
> >              /* KVM-HV has transactional memory on POWER8 also without the
> > -             * KVM_CAP_PPC_HTM extension, so enable it here instead. */
> > -            cap_htm = true;
> > +             * KVM_CAP_PPC_HTM extension, so enable it here instead as
> > +             * long as it's availble to userspace on the host. */
> > +            if (qemu_getauxval(AT_HWCAP2) & PPC_FEATURE2_HAS_HTM) {
> > +                cap_htm = true;
> > +            }
> 
> That's a very good idea! ... but I think you could also merge the two
> if-statements into one to save one level of indentation.
> 
>  Thomas