From: Suraj Jitindar Singh sjitindarsingh@gmail.com
commit 63279eeb7f93abb1692573c26f1e038e1a87358b upstream.
The performance monitoring unit (PMU) registers are saved on guest exit when the guest has set the pmcregs_in_use flag in its lppaca, if it exists, or unconditionally if it doesn't. If a nested guest is being run then the hypervisor doesn't, and in most cases can't, know if the PMU registers are in use since it doesn't know the location of the lppaca for the nested guest, although it may have one for its immediate guest. This results in the values of these registers being lost across nested guest entry and exit in the case where the nested guest was making use of the performance monitoring facility while it's nested guest hypervisor wasn't.
Further more the hypervisor could interrupt a guest hypervisor between when it has loaded up the PMU registers and it calling H_ENTER_NESTED or between returning from the nested guest to the guest hypervisor and the guest hypervisor reading the PMU registers, in kvmhv_p9_guest_entry(). This means that it isn't sufficient to just save the PMU registers when entering or exiting a nested guest, but that it is necessary to always save the PMU registers whenever a guest is capable of running nested guests to ensure the register values aren't lost in the context switch.
Ensure the PMU register values are preserved by always saving their value into the vcpu struct when a guest is capable of running nested guests.
This should have minimal performance impact however any impact can be avoided by booting a guest with "-machine pseries,cap-nested-hv=false" on the qemu commandline.
Fixes: 95a6432ce903 ("KVM: PPC: Book3S HV: Streamlined guest entry/exit path on P9 for radix guests") Cc: stable@vger.kernel.org # v4.20+ Signed-off-by: Suraj Jitindar Singh sjitindarsingh@gmail.com Signed-off-by: Michael Ellerman mpe@ellerman.id.au Link: https://lore.kernel.org/r/20190703012022.15644-1-sjitindarsingh@gmail.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- arch/powerpc/kvm/book3s_hv.c | 2 ++ 1 file changed, 2 insertions(+)
--- a/arch/powerpc/kvm/book3s_hv.c +++ b/arch/powerpc/kvm/book3s_hv.c @@ -3654,6 +3654,8 @@ int kvmhv_p9_guest_entry(struct kvm_vcpu vcpu->arch.vpa.dirty = 1; save_pmu = lp->pmcregs_in_use; } + /* Must save pmu if this guest is capable of running nested guests */ + save_pmu |= nesting_enabled(vcpu->kvm);
kvmhv_save_guest_pmu(vcpu, save_pmu);