Re: [PATCH v5 06/10] powerpc64/ftrace: Disable ftrace during kvm entry/exit

[Naveen N. Rao]

Steven Rostedt wrote:

On Thu, 19 Apr 2018 12:34:05 +0530
"Naveen N. Rao"  wrote:


2. If we are a secondary thread in Power8, then we would be in nap due
to SMT being disabled. We are woken up by an IPI to enter the guest. In
this scenario, we enter the guest through kvm_start_guest(). We disable
ftrace at this point. In this scenario, ftrace would only get re-enabled
on the secondary thread when SMT is re-enabled (via start_secondary()).


trace_hardirqs_off();

diff --git a/arch/powerpc/kvm/book3s_hv_rmhandlers.S 
b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
index bd63fa8a08b5..2c3cbe0067b2 100644
--- a/arch/powerpc/kvm/book3s_hv_rmhandlers.S
+++ b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
@@ -342,6 +342,9 @@ kvm_start_guest:
 
 	ld	r2,PACATOC(r13)
 


You may want to add a comment here about where ftrace gets re-enabled.


Sure. That would be:

/*
* If this is the primary thread, ftrace will get re-enabled when we
* go back to the hypervisor in kvmppc_run_core(). For secondary threads 
* on Power8, ftrace will get enabled when SMT is re-enabled through the 
* start_secondary() cpu bringup path.

*/

- Naveen

Re: [PATCH v5 06/10] powerpc64/ftrace: Disable ftrace during kvm entry/exit

[Steven Rostedt]

On Thu, 19 Apr 2018 12:34:05 +0530
"Naveen N. Rao"  wrote:

> 2. If we are a secondary thread in Power8, then we would be in nap due
> to SMT being disabled. We are woken up by an IPI to enter the guest. In
> this scenario, we enter the guest through kvm_start_guest(). We disable
> ftrace at this point. In this scenario, ftrace would only get re-enabled
> on the secondary thread when SMT is re-enabled (via start_secondary()).
> 
trace_hardirqs_off();
> diff --git a/arch/powerpc/kvm/book3s_hv_rmhandlers.S 
> b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
> index bd63fa8a08b5..2c3cbe0067b2 100644
> --- a/arch/powerpc/kvm/book3s_hv_rmhandlers.S
> +++ b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
> @@ -342,6 +342,9 @@ kvm_start_guest:
>  
>   ld  r2,PACATOC(r13)
>  

You may want to add a comment here about where ftrace gets re-enabled.

-- Steve

> + li  r0,0
> + stb r0,PACA_FTRACE_ENABLED(r13)
> +
>   li  r0,KVM_HWTHREAD_IN_KVM
>   stb r0,HSTATE_HWTHREAD_STATE(r13)
>  

[PATCH v5 06/10] powerpc64/ftrace: Disable ftrace during kvm entry/exit

[Naveen N. Rao]

During guest entry/exit, we switch over to/from the guest MMU context
and we cannot take exceptions in the hypervisor code.

Since ftrace may be enabled and since it can result in us taking a trap,
disable ftrace by setting paca->ftrace_enabled to zero. There are two
paths through which we enter/exit a guest:
1. If we are the vcore runner, then we enter the guest via
__kvmppc_vcore_entry() and we disable ftrace around this. This is always
the case for Power9, and for the primary thread on Power8.
2. If we are a secondary thread in Power8, then we would be in nap due
to SMT being disabled. We are woken up by an IPI to enter the guest. In
this scenario, we enter the guest through kvm_start_guest(). We disable
ftrace at this point. In this scenario, ftrace would only get re-enabled
on the secondary thread when SMT is re-enabled (via start_secondary()).

Signed-off-by: Naveen N. Rao 
---
 arch/powerpc/kvm/book3s_hv.c| 4 
 arch/powerpc/kvm/book3s_hv_rmhandlers.S | 3 +++
 2 files changed, 7 insertions(+)

diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index 4d07fca5121c..f604cbd8fc34 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -2911,8 +2911,12 @@ static noinline void kvmppc_run_core(struct kvmppc_vcore 
*vc)
 
srcu_idx = srcu_read_lock(>kvm->srcu);
 
+   this_cpu_disable_ftrace();
+
trap = __kvmppc_vcore_entry();
 
+   this_cpu_enable_ftrace();
+
srcu_read_unlock(>kvm->srcu, srcu_idx);
 
trace_hardirqs_off();
diff --git a/arch/powerpc/kvm/book3s_hv_rmhandlers.S 
b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
index bd63fa8a08b5..2c3cbe0067b2 100644
--- a/arch/powerpc/kvm/book3s_hv_rmhandlers.S
+++ b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
@@ -342,6 +342,9 @@ kvm_start_guest:
 
ld  r2,PACATOC(r13)
 
+   li  r0,0
+   stb r0,PACA_FTRACE_ENABLED(r13)
+
li  r0,KVM_HWTHREAD_IN_KVM
stb r0,HSTATE_HWTHREAD_STATE(r13)
 
-- 
2.17.0