[PATCH 1/3] KVM: PPC: Book3S HV: Always save guest pmu for guest capable of nesting

[Michael Ellerman]

Suraj Jitindar Singh <sjitindarsingh at gmail.com> writes:
> 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"

I'm not clear why this and the next commit are marked as fixing the
above commit. Wasn't it broken prior to that commit as well?

cheers

> Signed-off-by: Suraj Jitindar Singh <sjitindarsingh at gmail.com>
> ---
>  arch/powerpc/kvm/book3s_hv.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> index ec1804f822af..b682a429f3ef 100644
> --- 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, u64 time_limit,
>  		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);
>  
> -- 
> 2.13.6