Re: [Qemu-devel] [RFC v3 13/13] softmmu_template.h: move to multithreading

[Alex Bennée]

Alvise Rigo <address@hidden> writes:

> Exploiting the tcg_excl_access_lock, port the helper_{le,be}_st_name to
> work in real multithreading.
>
> - The macro lookup_cpus_ll_addr now uses directly the
>   env->excl_protected_addr to invalidate others' LL/SC operations
>
> Suggested-by: Jani Kokkonen <address@hidden>
> Suggested-by: Claudio Fontana <address@hidden>
> Signed-off-by: Alvise Rigo <address@hidden>
> ---
>  softmmu_template.h | 110 
> +++++++++++++++++++++++++++++++++++++++++++----------
>  1 file changed, 89 insertions(+), 21 deletions(-)
>
> diff --git a/softmmu_template.h b/softmmu_template.h
> index bc767f6..522454f 100644
> --- a/softmmu_template.h
> +++ b/softmmu_template.h
> @@ -141,21 +141,24 @@
>      vidx >= 0;                                                               
>  \
>  })
>  
> +#define EXCLUSIVE_RESET_ADDR ULLONG_MAX
> +
> +/* This macro requires the caller to have the tcg_excl_access_lock lock since
> + * it modifies the excl_protected_hwaddr of a running vCPU.
> + * The macros scans all the excl_protected_hwaddr of all the vCPUs and 
> compare
> + * them with the address the current vCPU is writing to. If there is a match,
> + * we reset the value, making the SC fail. */

It would have been nice if we had started with a comment when the
function^H^H^H^H^H macro was first introduced and then updated here.

>  #define lookup_cpus_ll_addr(addr)                                            
>  \
>  ({                                                                           
>  \
>      CPUState *cpu;                                                           
>  \
>      CPUArchState *acpu;                                                      
>  \
> -    bool hit = false;                                                        
>  \
>                                                                               
>  \
>      CPU_FOREACH(cpu) {                                                       
>  \
>          acpu = (CPUArchState *)cpu->env_ptr;                                 
>  \
>          if (cpu != current_cpu && acpu->excl_protected_hwaddr == addr) {     
>  \
> -            hit = true;                                                      
>  \
> -            break;                                                           
>  \
> +            acpu->excl_protected_hwaddr = EXCLUSIVE_RESET_ADDR;              
>  \
>          }                                                                    
>  \
>      }                                                                        
>  \
> -                                                                             
>  \
> -    hit;                                                                     
>  \
>  })

My comment about using an inline function in the earlier patch stands.

>  
>  #ifndef SOFTMMU_CODE_ACCESS
> @@ -439,18 +442,52 @@ void helper_le_st_name(CPUArchState *env, target_ulong 
> addr, DATA_TYPE val,
>               * exclusive-protected memory. */
>              hwaddr hw_addr = (iotlbentry->addr & TARGET_PAGE_MASK) + addr;
>  
> -            bool set_to_dirty;
> -
>              /* Two cases of invalidation: the current vCPU is writing to 
> another
>               * vCPU's exclusive address or the vCPU that issued the LoadLink 
> is
>               * writing to it, but not through a StoreCond. */
> -            set_to_dirty = lookup_cpus_ll_addr(hw_addr);
> -            set_to_dirty |= env->ll_sc_context &&
> -                           (env->excl_protected_hwaddr == hw_addr);
> +            qemu_mutex_lock(&tcg_excl_access_lock);
> +
> +            /* The macro lookup_cpus_ll_addr could have reset the exclusive
> +             * address. Fail the SC in this case.
> +             * N.B.: Here excl_succeeded == 0 means that we don't come from a
> +             * store conditional.  */
> +            if (env->excl_succeeded &&
> +                        (env->excl_protected_hwaddr == 
> EXCLUSIVE_RESET_ADDR)) {
> +                env->excl_succeeded = 0;
> +                qemu_mutex_unlock(&tcg_excl_access_lock);
> +
> +                return;
> +            }
> +
> +            lookup_cpus_ll_addr(hw_addr);

Add comment for side effect, also confused by the above comment given we
call lookups_cpus_ll_addr() after the comment about it tweaking excl_succedded.

> +
> +            if (!env->excl_succeeded) {
> +                if (env->ll_sc_context &&
> +                            (env->excl_protected_hwaddr == hw_addr)) {
> +                    cpu_physical_memory_set_excl_dirty(hw_addr);
> +                }
> +            } else {
> +                if (cpu_physical_memory_excl_is_dirty(hw_addr) ||
> +                        env->excl_protected_hwaddr != hw_addr) {
> +                    env->excl_protected_hwaddr = EXCLUSIVE_RESET_ADDR;
> +                    qemu_mutex_unlock(&tcg_excl_access_lock);
> +                    env->excl_succeeded = 0;
> +
> +                    return;
> +                }
> +            }

I'm wondering if it can be more naturally written the other way round to
aid comprehension:

if (env->excl_succeeded) {
   if (cpu_physical_memory_excl_is_dirty(hw_addr) ||
       env->excl_protected_hwaddr != hw_addr) {
       ..do stuff..
       return
   }
} else {
   if (env->ll_sc_context &&
      (env->excl_protected_hwaddr == hw_addr)) {
      cpu_physical_memory_set_excl_dirty(hw_addr);
   }
}

Although now I'm confused as to why we push on in 3 of the 4 cases.

> +
> +            haddr = addr + env->tlb_table[mmu_idx][index].addend;
> +        #if DATA_SIZE == 1
> +            glue(glue(st, SUFFIX), _p)((uint8_t *)haddr, val);
> +        #else
> +            glue(glue(st, SUFFIX), _le_p)((uint8_t *)haddr, val);
> +        #endif
> +
> +            env->excl_protected_hwaddr = EXCLUSIVE_RESET_ADDR;
> +            qemu_mutex_unlock(&tcg_excl_access_lock);
>  
> -            if (set_to_dirty) {
> -                cpu_physical_memory_set_excl_dirty(hw_addr);
> -            } /* the vCPU is legitimately writing to the protected address */
> +            return;
>          } else {
>              if ((addr & (DATA_SIZE - 1)) != 0) {
>                  goto do_unaligned_access;
> @@ -537,18 +574,49 @@ void helper_be_st_name(CPUArchState *env, target_ulong 
> addr, DATA_TYPE val,
>               * exclusive-protected memory. */
>              hwaddr hw_addr = (iotlbentry->addr & TARGET_PAGE_MASK) + addr;
>  
> -            bool set_to_dirty;
> -
>              /* Two cases of invalidation: the current vCPU is writing to 
> another
>               * vCPU's exclusive address or the vCPU that issued the LoadLink 
> is
>               * writing to it, but not through a StoreCond. */
> -            set_to_dirty = lookup_cpus_ll_addr(hw_addr);
> -            set_to_dirty |= env->ll_sc_context &&
> -                           (env->excl_protected_hwaddr == hw_addr);
> +            qemu_mutex_lock(&tcg_excl_access_lock);
> +
> +            /* The macro lookup_cpus_ll_addr could have reset the exclusive
> +             * address. Fail the SC in this case.
> +             * N.B.: Here excl_succeeded == 0 means that we don't come from a
> +             * store conditional.  */
> +            if (env->excl_succeeded &&
> +                        (env->excl_protected_hwaddr == 
> EXCLUSIVE_RESET_ADDR)) {
> +                env->excl_succeeded = 0;
> +                qemu_mutex_unlock(&tcg_excl_access_lock);
> +
> +                return;
> +            }
> +
> +            lookup_cpus_ll_addr(hw_addr);
> +
> +            if (!env->excl_succeeded) {
> +                if (env->ll_sc_context &&
> +                            (env->excl_protected_hwaddr == hw_addr)) {
> +                    cpu_physical_memory_set_excl_dirty(hw_addr);
> +                }
> +            } else {
> +                if (cpu_physical_memory_excl_is_dirty(hw_addr) ||
> +                        env->excl_protected_hwaddr != hw_addr) {
> +                    env->excl_protected_hwaddr = EXCLUSIVE_RESET_ADDR;
> +                    qemu_mutex_unlock(&tcg_excl_access_lock);
> +                    env->excl_succeeded = 0;
> +
> +                    return;
> +                }
> +            }

Given the amount of copy/paste between the two functions I wonder if
there is some commonality to be re-factored out here?

>  
> -            if (set_to_dirty) {
> -                cpu_physical_memory_set_excl_dirty(hw_addr);
> -            } /* the vCPU is legitimately writing to the protected address */
> +            haddr = addr + env->tlb_table[mmu_idx][index].addend;
> +
> +            glue(glue(st, SUFFIX), _be_p)((uint8_t *)haddr, val);
> +
> +            qemu_mutex_unlock(&tcg_excl_access_lock);
> +            env->excl_protected_hwaddr = EXCLUSIVE_RESET_ADDR;
> +
> +            return;
>          } else {
>              if ((addr & (DATA_SIZE - 1)) != 0) {
>                  goto do_unaligned_access;

-- 
Alex Bennée