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

[Thomas Huth]

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?

> 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