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

[David Gibson]

On Thu, Mar 30, 2017 at 01:28:36PM +1100, Sam Bobroff wrote:
> 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 ;-)

Ok, given that, I'm moving this fix from ppc-for-2.9 to ppc-for-2.10.

> 
> > > 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 <address@hidden>
> > > ---
> > >  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
> 

-- 
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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