Re: [Qemu-devel] [RFC v1 09/11] tcg: drop global lock during TCG code execution

[KONRAD Frederic <fred.konrad@greensocs.com>]

Hi Alex,

Thanks for having pulling all that together!
About this patch the original author was Jan Kiszka 
(https://lists.gnu.org/archive/html/qemu-devel/2011-09/msg03323.html)

This has probably been dropped during a rebase.

Thanks,
Fred

Le 18/03/2016 17:18, Alex Bennée a écrit :
> From: KONRAD Frederic <fred.konrad@greensocs.com>
>
> This finally allows TCG to benefit from the iothread introduction: Drop
> the global mutex while running pure TCG CPU code. Reacquire the lock
> when entering MMIO or PIO emulation, or when leaving the TCG loop.
>
> We have to revert a few optimization for the current TCG threading
> model, namely kicking the TCG thread in qemu_mutex_lock_iothread and not
> kicking it in qemu_cpu_kick. We also need to disable RAM block
> reordering until we have a more efficient locking mechanism at hand.
>
> Still, a Linux x86 UP guest and my Musicpal ARM model boot fine here.
> These numbers demonstrate where we gain something:
>
> 20338 jan       20   0  331m  75m 6904 R   99  0.9   0:50.95 qemu-system-arm
> 20337 jan       20   0  331m  75m 6904 S   20  0.9   0:26.50 qemu-system-arm
>
> The guest CPU was fully loaded, but the iothread could still run mostly
> independent on a second core. Without the patch we don't get beyond
>
> 32206 jan       20   0  330m  73m 7036 R   82  0.9   1:06.00 qemu-system-arm
> 32204 jan       20   0  330m  73m 7036 S   21  0.9   0:17.03 qemu-system-arm
>
> We don't benefit significantly, though, when the guest is not fully
> loading a host CPU.
>
> Message-Id: <1439220437-23957-10-git-send-email-fred.konrad@greensocs.com>
> Signed-off-by: KONRAD Frederic <fred.konrad@greensocs.com>
> [AJB: -smp single-threaded fix, rm old info from commit msg]
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
>
> ---
> v1 (ajb):
>    - SMP failure now fixed by previous commit
> Changes from Fred Konrad (mttcg-v7 via paolo):
>    * Rebase on the current HEAD.
>    * Fixes a deadlock in qemu_devices_reset().
>    * Remove the mutex in address_space_*
> ---
>   cpu-exec.c         | 10 ++++++++++
>   cpus.c             | 26 +++-----------------------
>   cputlb.c           |  4 ++++
>   exec.c             | 12 ++++++++++++
>   hw/i386/kvmvapic.c |  3 +++
>   softmmu_template.h | 17 +++++++++++++++++
>   translate-all.c    | 11 +++++++++--
>   7 files changed, 58 insertions(+), 25 deletions(-)
>
> diff --git a/cpu-exec.c b/cpu-exec.c
> index 3572256..76891fd 100644
> --- a/cpu-exec.c
> +++ b/cpu-exec.c
> @@ -28,6 +28,7 @@
>   #include "qemu/rcu.h"
>   #include "exec/tb-hash.h"
>   #include "exec/log.h"
> +#include "qemu/main-loop.h"
>   #if defined(TARGET_I386) && !defined(CONFIG_USER_ONLY)
>   #include "hw/i386/apic.h"
>   #endif
> @@ -449,6 +450,13 @@ int cpu_exec(CPUState *cpu)
>               for(;;) {
>                   interrupt_request = cpu->interrupt_request;
>                   if (unlikely(interrupt_request)) {
> +                    /* FIXME: this needs to take the iothread lock.
> +                     * For this we need to find all places in
> +                     * cc->cpu_exec_interrupt that can call cpu_loop_exit,
> +                     * and call qemu_unlock_iothread_mutex() there.  Else,
> +                     * add a flag telling cpu_loop_exit() to unlock it.
> +                     */
> +
>                       if (unlikely(cpu->singlestep_enabled & SSTEP_NOIRQ)) {
>                           /* Mask out external interrupts for this step. */
>                           interrupt_request &= ~CPU_INTERRUPT_SSTEP_MASK;
> @@ -501,6 +509,7 @@ int cpu_exec(CPUState *cpu)
>                              the program flow was changed */
>                           next_tb = 0;
>                       }
> +
>                   }
>                   if (unlikely(cpu->exit_request
>                                || replay_has_interrupt())) {
> @@ -618,6 +627,7 @@ int cpu_exec(CPUState *cpu)
>               g_assert(env == &x86_cpu->env);
>   #endif
>   #endif /* buggy compiler */
> +
>               cpu->can_do_io = 1;
>               tb_lock_reset();
>           }
> diff --git a/cpus.c b/cpus.c
> index a87fbf9..0e3d5f9 100644
> --- a/cpus.c
> +++ b/cpus.c
> @@ -911,8 +911,6 @@ static void qemu_kvm_init_cpu_signals(CPUState *cpu)
>   #endif /* _WIN32 */
>   
>   static QemuMutex qemu_global_mutex;
> -static QemuCond qemu_io_proceeded_cond;
> -static unsigned iothread_requesting_mutex;
>   
>   static QemuThread io_thread;
>   
> @@ -928,7 +926,6 @@ void qemu_init_cpu_loop(void)
>       qemu_cond_init(&qemu_cpu_cond);
>       qemu_cond_init(&qemu_pause_cond);
>       qemu_cond_init(&qemu_work_cond);
> -    qemu_cond_init(&qemu_io_proceeded_cond);
>       qemu_mutex_init(&qemu_global_mutex);
>   
>       qemu_thread_get_self(&io_thread);
> @@ -1043,10 +1040,6 @@ static void qemu_tcg_wait_io_event(CPUState *cpu)
>           qemu_cond_wait(cpu->halt_cond, &qemu_global_mutex);
>       }
>   
> -    while (iothread_requesting_mutex) {
> -        qemu_cond_wait(&qemu_io_proceeded_cond, &qemu_global_mutex);
> -    }
> -
>       CPU_FOREACH(cpu) {
>           qemu_wait_io_event_common(cpu);
>       }
> @@ -1314,22 +1307,7 @@ bool qemu_mutex_iothread_locked(void)
>   
>   void qemu_mutex_lock_iothread(void)
>   {
> -    atomic_inc(&iothread_requesting_mutex);
> -    /* In the simple case there is no need to bump the VCPU thread out of
> -     * TCG code execution.
> -     */
> -    if (!tcg_enabled() || qemu_in_vcpu_thread() ||
> -        !first_cpu || !first_cpu->created) {
> -        qemu_mutex_lock(&qemu_global_mutex);
> -        atomic_dec(&iothread_requesting_mutex);
> -    } else {
> -        if (qemu_mutex_trylock(&qemu_global_mutex)) {
> -            qemu_cpu_kick_no_halt();
> -            qemu_mutex_lock(&qemu_global_mutex);
> -        }
> -        atomic_dec(&iothread_requesting_mutex);
> -        qemu_cond_broadcast(&qemu_io_proceeded_cond);
> -    }
> +    qemu_mutex_lock(&qemu_global_mutex);
>       iothread_locked = true;
>   }
>   
> @@ -1575,7 +1553,9 @@ static int tcg_cpu_exec(CPUState *cpu)
>           cpu->icount_decr.u16.low = decr;
>           cpu->icount_extra = count;
>       }
> +    qemu_mutex_unlock_iothread();
>       ret = cpu_exec(cpu);
> +    qemu_mutex_lock_iothread();
>   #ifdef CONFIG_PROFILER
>       tcg_time += profile_getclock() - ti;
>   #endif
> diff --git a/cputlb.c b/cputlb.c
> index 2f7a166..d607471 100644
> --- a/cputlb.c
> +++ b/cputlb.c
> @@ -30,6 +30,10 @@
>   #include "exec/ram_addr.h"
>   #include "tcg/tcg.h"
>   
> +#ifdef CONFIG_SOFTMMU
> +#include "qemu/main-loop.h"
> +#endif
> +
>   //#define DEBUG_TLB
>   //#define DEBUG_TLB_CHECK
>   
> diff --git a/exec.c b/exec.c
> index 402b751..9986d59 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -2113,6 +2113,12 @@ static void check_watchpoint(int offset, int len, MemTxAttrs attrs, int flags)
>                   }
>                   cpu->watchpoint_hit = wp;
>   
> +                /*
> +                 * Unlock iothread mutex before calling cpu_loop_exit
> +                 * or cpu_resume_from_signal.
> +                 */
> +                qemu_mutex_unlock_iothread();
> +
>                   /* Unlocked by cpu_loop_exit or cpu_resume_from_signal.  */
>                   tb_lock();
>                   tb_check_watchpoint(cpu);
> @@ -2348,8 +2354,14 @@ static void io_mem_init(void)
>       memory_region_init_io(&io_mem_rom, NULL, &unassigned_mem_ops, NULL, NULL, UINT64_MAX);
>       memory_region_init_io(&io_mem_unassigned, NULL, &unassigned_mem_ops, NULL,
>                             NULL, UINT64_MAX);
> +
> +    /* io_mem_notdirty calls tb_invalidate_phys_page_fast,
> +     * which must be called without the iothread mutex.
> +     */
>       memory_region_init_io(&io_mem_notdirty, NULL, &notdirty_mem_ops, NULL,
>                             NULL, UINT64_MAX);
> +    memory_region_clear_global_locking(&io_mem_notdirty);
> +
>       memory_region_init_io(&io_mem_watch, NULL, &watch_mem_ops, NULL,
>                             NULL, UINT64_MAX);
>   }
> diff --git a/hw/i386/kvmvapic.c b/hw/i386/kvmvapic.c
> index 7c0d542..a0439a8 100644
> --- a/hw/i386/kvmvapic.c
> +++ b/hw/i386/kvmvapic.c
> @@ -447,6 +447,9 @@ static void patch_instruction(VAPICROMState *s, X86CPU *cpu, target_ulong ip)
>       resume_all_vcpus();
>   
>       if (!kvm_enabled()) {
> +        /* Unlock iothread mutex before calling cpu_resume_from_signal.  */
> +        qemu_mutex_unlock_iothread();
> +
>           /* Unlocked by cpu_resume_from_signal.  */
>           tb_lock();
>           cs->current_tb = NULL;
> diff --git a/softmmu_template.h b/softmmu_template.h
> index 208f808..0b6c609 100644
> --- a/softmmu_template.h
> +++ b/softmmu_template.h
> @@ -151,6 +151,7 @@ static inline DATA_TYPE glue(io_read, SUFFIX)(CPUArchState *env,
>       CPUState *cpu = ENV_GET_CPU(env);
>       hwaddr physaddr = iotlbentry->addr;
>       MemoryRegion *mr = iotlb_to_region(cpu, physaddr, iotlbentry->attrs);
> +    bool locked = false;
>   
>       physaddr = (physaddr & TARGET_PAGE_MASK) + addr;
>       cpu->mem_io_pc = retaddr;
> @@ -159,8 +160,15 @@ static inline DATA_TYPE glue(io_read, SUFFIX)(CPUArchState *env,
>       }
>   
>       cpu->mem_io_vaddr = addr;
> +    if (mr->global_locking) {
> +        qemu_mutex_lock_iothread();
> +        locked = true;
> +    }
>       memory_region_dispatch_read(mr, physaddr, &val, 1 << SHIFT,
>                                   iotlbentry->attrs);
> +    if (locked) {
> +        qemu_mutex_unlock_iothread();
> +    }
>       return val;
>   }
>   #endif
> @@ -358,6 +366,7 @@ static inline void glue(io_write, SUFFIX)(CPUArchState *env,
>       CPUState *cpu = ENV_GET_CPU(env);
>       hwaddr physaddr = iotlbentry->addr;
>       MemoryRegion *mr = iotlb_to_region(cpu, physaddr, iotlbentry->attrs);
> +    bool locked = false;
>   
>       physaddr = (physaddr & TARGET_PAGE_MASK) + addr;
>       if (mr != &io_mem_rom && mr != &io_mem_notdirty && !cpu->can_do_io) {
> @@ -366,8 +375,16 @@ static inline void glue(io_write, SUFFIX)(CPUArchState *env,
>   
>       cpu->mem_io_vaddr = addr;
>       cpu->mem_io_pc = retaddr;
> +
> +    if (mr->global_locking) {
> +        qemu_mutex_lock_iothread();
> +        locked = true;
> +    }
>       memory_region_dispatch_write(mr, physaddr, val, 1 << SHIFT,
>                                    iotlbentry->attrs);
> +    if (locked) {
> +        qemu_mutex_unlock_iothread();
> +    }
>   }
>   
>   void helper_le_st_name(CPUArchState *env, target_ulong addr, DATA_TYPE val,
> diff --git a/translate-all.c b/translate-all.c
> index 1aab243..fe1392a 100644
> --- a/translate-all.c
> +++ b/translate-all.c
> @@ -1221,6 +1221,7 @@ void tb_invalidate_phys_range(tb_page_addr_t start, tb_page_addr_t end)
>    * this TB.
>    *
>    * Called with mmap_lock held for user-mode emulation
> + * If called from generated code, iothread mutex must not be held.
>    */
>   void tb_invalidate_phys_page_range(tb_page_addr_t start, tb_page_addr_t end,
>                                      int is_cpu_write_access)
> @@ -1333,7 +1334,9 @@ void tb_invalidate_phys_page_range(tb_page_addr_t start, tb_page_addr_t end,
>   }
>   
>   #ifdef CONFIG_SOFTMMU
> -/* len must be <= 8 and start must be a multiple of len */
> +/* len must be <= 8 and start must be a multiple of len.
> + * Called via softmmu_template.h, with iothread mutex not held.
> + */
>   void tb_invalidate_phys_page_fast(tb_page_addr_t start, int len)
>   {
>       PageDesc *p;
> @@ -1591,6 +1594,7 @@ static TranslationBlock *tb_find_pc(uintptr_t tc_ptr)
>   }
>   
>   #if !defined(CONFIG_USER_ONLY)
> +/* If called from generated code, iothread mutex must not be held.  */
>   void tb_invalidate_phys_addr(AddressSpace *as, hwaddr addr)
>   {
>       ram_addr_t ram_addr;
> @@ -1637,7 +1641,10 @@ void tb_check_watchpoint(CPUState *cpu)
>   
>   #ifndef CONFIG_USER_ONLY
>   /* in deterministic execution mode, instructions doing device I/Os
> -   must be at the end of the TB */
> + * must be at the end of the TB.
> + *
> + * Called by softmmu_template.h, with iothread mutex not held.
> + */
>   void cpu_io_recompile(CPUState *cpu, uintptr_t retaddr)
>   {
>   #if defined(TARGET_MIPS) || defined(TARGET_SH4)