When switching a new vCPU on we want to complete a bunch of the setup
work before we start scheduling the vCPU thread. To do this cleanly we
defer vCPU setup to async work which will run the vCPUs execution
context as the thread is woken up. The scheduling of the work will kick
the vCPU awake.

This avoids potential races in MTTCG system emulation.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>

---
v7
  - add const to static mode_for_el[] array
  - fix checkpatch long lines
---
 target/arm/arm-powerctl.c | 146 ++++++++++++++++++++++++++++------------------
 1 file changed, 88 insertions(+), 58 deletions(-)

diff --git a/target/arm/arm-powerctl.c b/target/arm/arm-powerctl.c
index fbb7a15daa..082788e3a4 100644
--- a/target/arm/arm-powerctl.c
+++ b/target/arm/arm-powerctl.c
@@ -48,11 +48,87 @@ CPUState *arm_get_cpu_by_id(uint64_t id)
     return NULL;
 }
 
+struct cpu_on_info {
+    uint64_t entry;
+    uint64_t context_id;
+    uint32_t target_el;
+    bool target_aa64;
+};
+
+
+static void arm_set_cpu_on_async_work(CPUState *target_cpu_state,
+                                      run_on_cpu_data data)
+{
+    ARMCPU *target_cpu = ARM_CPU(target_cpu_state);
+    struct cpu_on_info *info = (struct cpu_on_info *) data.host_ptr;
+
+    /* Initialize the cpu we are turning on */
+    cpu_reset(target_cpu_state);
+    target_cpu->powered_off = false;
+    target_cpu_state->halted = 0;
+
+    if (info->target_aa64) {
+        if ((info->target_el < 3) && arm_feature(&target_cpu->env,
+                                                 ARM_FEATURE_EL3)) {
+            /*
+             * As target mode is AArch64, we need to set lower
+             * exception level (the requested level 2) to AArch64
+             */
+            target_cpu->env.cp15.scr_el3 |= SCR_RW;
+        }
+
+        if ((info->target_el < 2) && arm_feature(&target_cpu->env,
+                                                 ARM_FEATURE_EL2)) {
+            /*
+             * As target mode is AArch64, we need to set lower
+             * exception level (the requested level 1) to AArch64
+             */
+            target_cpu->env.cp15.hcr_el2 |= HCR_RW;
+        }
+
+        target_cpu->env.pstate = aarch64_pstate_mode(info->target_el, true);
+    } else {
+        /* We are requested to boot in AArch32 mode */
+        static const uint32_t mode_for_el[] = { 0,
+                                                ARM_CPU_MODE_SVC,
+                                                ARM_CPU_MODE_HYP,
+                                                ARM_CPU_MODE_SVC };
+
+        cpsr_write(&target_cpu->env, mode_for_el[info->target_el], CPSR_M,
+                   CPSRWriteRaw);
+    }
+
+    if (info->target_el == 3) {
+        /* Processor is in secure mode */
+        target_cpu->env.cp15.scr_el3 &= ~SCR_NS;
+    } else {
+        /* Processor is not in secure mode */
+        target_cpu->env.cp15.scr_el3 |= SCR_NS;
+    }
+
+    /* We check if the started CPU is now at the correct level */
+    assert(info->target_el == arm_current_el(&target_cpu->env));
+
+    if (info->target_aa64) {
+        target_cpu->env.xregs[0] = info->context_id;
+        target_cpu->env.thumb = false;
+    } else {
+        target_cpu->env.regs[0] = info->context_id;
+        target_cpu->env.thumb = info->entry & 1;
+        info->entry &= 0xfffffffe;
+    }
+
+    /* Start the new CPU at the requested address */
+    cpu_set_pc(target_cpu_state, info->entry);
+    g_free(info);
+}
+
 int arm_set_cpu_on(uint64_t cpuid, uint64_t entry, uint64_t context_id,
                    uint32_t target_el, bool target_aa64)
 {
     CPUState *target_cpu_state;
     ARMCPU *target_cpu;
+    struct cpu_on_info *info;
 
     DPRINTF("cpu %" PRId64 " (EL %d, %s) @ 0x%" PRIx64 " with R0 = 0x%" PRIx64
             "\n", cpuid, target_el, target_aa64 ? "aarch64" : "aarch32", entry,
@@ -109,64 +185,18 @@ int arm_set_cpu_on(uint64_t cpuid, uint64_t entry, uint64_t context_id,
         return QEMU_ARM_POWERCTL_INVALID_PARAM;
     }
 
-    /* Initialize the cpu we are turning on */
-    cpu_reset(target_cpu_state);
-    target_cpu->powered_off = false;
-    target_cpu_state->halted = 0;
-
-    if (target_aa64) {
-        if ((target_el < 3) && arm_feature(&target_cpu->env, ARM_FEATURE_EL3)) {
-            /*
-             * As target mode is AArch64, we need to set lower
-             * exception level (the requested level 2) to AArch64
-             */
-            target_cpu->env.cp15.scr_el3 |= SCR_RW;
-        }
-
-        if ((target_el < 2) && arm_feature(&target_cpu->env, ARM_FEATURE_EL2)) {
-            /*
-             * As target mode is AArch64, we need to set lower
-             * exception level (the requested level 1) to AArch64
-             */
-            target_cpu->env.cp15.hcr_el2 |= HCR_RW;
-        }
-
-        target_cpu->env.pstate = aarch64_pstate_mode(target_el, true);
-    } else {
-        /* We are requested to boot in AArch32 mode */
-        static uint32_t mode_for_el[] = { 0,
-                                          ARM_CPU_MODE_SVC,
-                                          ARM_CPU_MODE_HYP,
-                                          ARM_CPU_MODE_SVC };
-
-        cpsr_write(&target_cpu->env, mode_for_el[target_el], CPSR_M,
-                   CPSRWriteRaw);
-    }
-
-    if (target_el == 3) {
-        /* Processor is in secure mode */
-        target_cpu->env.cp15.scr_el3 &= ~SCR_NS;
-    } else {
-        /* Processor is not in secure mode */
-        target_cpu->env.cp15.scr_el3 |= SCR_NS;
-    }
-
-    /* We check if the started CPU is now at the correct level */
-    assert(target_el == arm_current_el(&target_cpu->env));
-
-    if (target_aa64) {
-        target_cpu->env.xregs[0] = context_id;
-        target_cpu->env.thumb = false;
-    } else {
-        target_cpu->env.regs[0] = context_id;
-        target_cpu->env.thumb = entry & 1;
-        entry &= 0xfffffffe;
-    }
-
-    /* Start the new CPU at the requested address */
-    cpu_set_pc(target_cpu_state, entry);
-
-    qemu_cpu_kick(target_cpu_state);
+    /* To avoid racing with a CPU we are just kicking off we do the
+     * final bit of preparation for the work in the target CPUs
+     * context.
+     */
+    info = g_new(struct cpu_on_info, 1);
+    info->entry = entry;
+    info->context_id = context_id;
+    info->target_el = target_el;
+    info->target_aa64 = target_aa64;
+
+    async_run_on_cpu(target_cpu_state, arm_set_cpu_on_async_work,
+                     RUN_ON_CPU_HOST_PTR(info));
 
     /* We are good to go */
     return QEMU_ARM_POWERCTL_RET_SUCCESS;
-- 
2.11.0

On 1 February 2017 at 15:05, Alex Bennée <alex.bennee@linaro.org> wrote:
> When switching a new vCPU on we want to complete a bunch of the setup
> work before we start scheduling the vCPU thread. To do this cleanly we
> defer vCPU setup to async work which will run the vCPUs execution
> context as the thread is woken up. The scheduling of the work will kick
> the vCPU awake.
>
> This avoids potential races in MTTCG system emulation.
>
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> Reviewed-by: Richard Henderson <rth@twiddle.net>

Can we now have races between arm_set_cpu_on() and
arm_set_cpu_off() ? It's not clear to me what prevents that.

With this change our PSCI CPU_ON is no longer effectively
atomic, which means we need to think about the races
between PSCI CPU_ON and CPU_OFF, and the fact that the
core might be in what the PSCI spec section 6.6
calls an ON_PENDING state (ie CPU_ON has been called
for it but it hasn't actually booted yet).

thanks
-- PMM

Peter Maydell <peter.maydell@linaro.org> writes:

> On 1 February 2017 at 15:05, Alex Bennée <alex.bennee@linaro.org> wrote:
>> When switching a new vCPU on we want to complete a bunch of the setup
>> work before we start scheduling the vCPU thread. To do this cleanly we
>> defer vCPU setup to async work which will run the vCPUs execution
>> context as the thread is woken up. The scheduling of the work will kick
>> the vCPU awake.
>>
>> This avoids potential races in MTTCG system emulation.
>>
>> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
>> Reviewed-by: Richard Henderson <rth@twiddle.net>
>
> Can we now have races between arm_set_cpu_on() and
> arm_set_cpu_off() ? It's not clear to me what prevents that.
>
> With this change our PSCI CPU_ON is no longer effectively
> atomic, which means we need to think about the races
> between PSCI CPU_ON and CPU_OFF, and the fact that the
> core might be in what the PSCI spec section 6.6
> calls an ON_PENDING state (ie CPU_ON has been called
> for it but it hasn't actually booted yet).

Would it be enough to also queue the set_cpu_off work?

The queue itself is safe to add to so you'll end up with a series of
on/off deferred work that will eventually unwind itself when the CPU
thread runs.

>
> thanks
> -- PMM


--
Alex Bennée