Currently, on a P10 KVM guest, the mitigations seen in the output of "lscpu" command are different from the host. The reason for this behaviour is that when the KVM guest makes the "h_get_cpu_characteristics" hcall, QEMU does not consider the data it received from the host via the KVM_PPC_GET_CPU_CHAR ioctl, and just uses the values present in spapr->eff.caps[], which in turn just contain the default values set in spapr_machine_class_init().
Fix this behaviour by making sure that h_get_cpu_characteristics() returns the data received from the KVM ioctl for a KVM guest. Perf impact: With null syscall benchmark[1], ~45% improvement is observed. 1. Vanilla QEMU $ ./null_syscall 132.19 ns 456.54 cycles 2. With this patch $ ./null_syscall 91.18 ns 314.57 cycles [1]: https://ozlabs.org/~anton/junkcode/null_syscall.c Signed-off-by: Gautam Menghani <gau...@linux.ibm.com> --- hw/ppc/spapr_hcall.c | 6 ++++++ include/hw/ppc/spapr.h | 1 + target/ppc/kvm.c | 2 ++ 3 files changed, 9 insertions(+) diff --git a/hw/ppc/spapr_hcall.c b/hw/ppc/spapr_hcall.c index 406aea4ecb..6aec4e22fc 100644 --- a/hw/ppc/spapr_hcall.c +++ b/hw/ppc/spapr_hcall.c @@ -1415,6 +1415,12 @@ static target_ulong h_get_cpu_characteristics(PowerPCCPU *cpu, uint8_t count_cache_flush_assist = spapr_get_cap(spapr, SPAPR_CAP_CCF_ASSIST); + if (kvm_enabled()) { + args[0] = spapr->chars.character; + args[1] = spapr->chars.behaviour; + return H_SUCCESS; + } + switch (safe_cache) { case SPAPR_CAP_WORKAROUND: characteristics |= H_CPU_CHAR_L1D_FLUSH_ORI30; diff --git a/include/hw/ppc/spapr.h b/include/hw/ppc/spapr.h index 39bd5bd5ed..b1e3ee1ae2 100644 --- a/include/hw/ppc/spapr.h +++ b/include/hw/ppc/spapr.h @@ -283,6 +283,7 @@ struct SpaprMachineState { Error *fwnmi_migration_blocker; SpaprWatchdog wds[WDT_MAX_WATCHDOGS]; + struct kvm_ppc_cpu_char chars; }; #define H_SUCCESS 0 diff --git a/target/ppc/kvm.c b/target/ppc/kvm.c index 992356cb75..fee6c5d131 100644 --- a/target/ppc/kvm.c +++ b/target/ppc/kvm.c @@ -2511,6 +2511,7 @@ bool kvmppc_has_cap_xive(void) static void kvmppc_get_cpu_characteristics(KVMState *s) { + SpaprMachineState *spapr = SPAPR_MACHINE(qdev_get_machine()); struct kvm_ppc_cpu_char c; int ret; @@ -2528,6 +2529,7 @@ static void kvmppc_get_cpu_characteristics(KVMState *s) return; } + spapr->chars = c; cap_ppc_safe_cache = parse_cap_ppc_safe_cache(c); cap_ppc_safe_bounds_check = parse_cap_ppc_safe_bounds_check(c); cap_ppc_safe_indirect_branch = parse_cap_ppc_safe_indirect_branch(c); -- 2.49.0
