Re: [Qemu-devel] [RFC v7 13/16] softmmu: Add history of excl accesses

["Alex Bennée" <alex.bennee@linaro.org>]

Alvise Rigo <a.rigo@virtualopensystems.com> writes:

> Add a circular buffer to store the hw addresses used in the last
> EXCLUSIVE_HISTORY_LEN exclusive accesses.
>
> When an address is pop'ed from the buffer, its page will be set as not
> exclusive. In this way, we avoid:
> - frequent set/unset of a page (causing frequent flushes as well)
> - the possibility to forget the EXCL bit set.

Why was this a possibility before? Shouldn't that be tackled in the
patch that introduced it?

>
> Suggested-by: Jani Kokkonen <jani.kokkonen@huawei.com>
> Suggested-by: Claudio Fontana <claudio.fontana@huawei.com>
> Signed-off-by: Alvise Rigo <a.rigo@virtualopensystems.com>
> ---
>  cputlb.c                | 29 +++++++++++++++++++----------
>  exec.c                  | 19 +++++++++++++++++++
>  include/qom/cpu.h       |  8 ++++++++
>  softmmu_llsc_template.h |  1 +
>  vl.c                    |  3 +++
>  5 files changed, 50 insertions(+), 10 deletions(-)
>
> diff --git a/cputlb.c b/cputlb.c
> index 06ce2da..f3c4d97 100644
> --- a/cputlb.c
> +++ b/cputlb.c
> @@ -395,16 +395,6 @@ void tlb_set_page_with_attrs(CPUState *cpu, target_ulong vaddr,
>      env->tlb_v_table[mmu_idx][vidx] = *te;
>      env->iotlb_v[mmu_idx][vidx] = env->iotlb[mmu_idx][index];
>
> -    if (unlikely(!(te->addr_write & TLB_MMIO) && (te->addr_write & TLB_EXCL))) {
> -        /* We are removing an exclusive entry, set the page to dirty. This
> -         * is not be necessary if the vCPU has performed both SC and LL. */
> -        hwaddr hw_addr = (env->iotlb[mmu_idx][index].addr & TARGET_PAGE_MASK) +
> -                                          (te->addr_write & TARGET_PAGE_MASK);
> -        if (!cpu->ll_sc_context) {
> -            cpu_physical_memory_unset_excl(hw_addr);
> -        }
> -    }
> -

Errm is this right? I got confused reviewing 8/16 because my final tree
didn't have this code. I'm not sure the adding of history obviates the
need to clear the exclusive flag?

>      /* refill the tlb */
>      env->iotlb[mmu_idx][index].addr = iotlb - vaddr;
>      env->iotlb[mmu_idx][index].attrs = attrs;
> @@ -517,6 +507,25 @@ static inline bool lookup_and_reset_cpus_ll_addr(hwaddr addr, hwaddr size)
>      return ret;
>  }
>
> +extern CPUExclusiveHistory excl_history;
> +static inline void excl_history_put_addr(hwaddr addr)
> +{
> +    hwaddr last;
> +
> +    /* Calculate the index of the next exclusive address */
> +    excl_history.last_idx = (excl_history.last_idx + 1) % excl_history.length;
> +
> +    last = excl_history.c_array[excl_history.last_idx];
> +
> +    /* Unset EXCL bit of the oldest entry */
> +    if (last != EXCLUSIVE_RESET_ADDR) {
> +        cpu_physical_memory_unset_excl(last);
> +    }
> +
> +    /* Add a new address, overwriting the oldest one */
> +    excl_history.c_array[excl_history.last_idx] = addr & TARGET_PAGE_MASK;
> +}
> +
>  #define MMUSUFFIX _mmu
>
>  /* Generates LoadLink/StoreConditional helpers in softmmu_template.h */
> diff --git a/exec.c b/exec.c
> index 51f366d..2e123f1 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -177,6 +177,25 @@ struct CPUAddressSpace {
>      MemoryListener tcg_as_listener;
>  };
>
> +/* Exclusive memory support */
> +CPUExclusiveHistory excl_history;
> +void cpu_exclusive_history_init(void)
> +{
> +    /* Initialize exclusive history for atomic instruction handling. */
> +    if (tcg_enabled()) {
> +        g_assert(EXCLUSIVE_HISTORY_CPU_LEN * max_cpus <= UINT16_MAX);
> +        excl_history.length = EXCLUSIVE_HISTORY_CPU_LEN * max_cpus;
> +        excl_history.c_array = g_malloc(excl_history.length * sizeof(hwaddr));
> +        memset(excl_history.c_array, -1, excl_history.length * sizeof(hwaddr));
> +    }
> +}
> +
> +void cpu_exclusive_history_free(void)
> +{
> +    if (tcg_enabled()) {
> +        g_free(excl_history.c_array);
> +    }
> +}
>  #endif
>
>  #if !defined(CONFIG_USER_ONLY)
> diff --git a/include/qom/cpu.h b/include/qom/cpu.h
> index 6f6c1c0..0452fd0 100644
> --- a/include/qom/cpu.h
> +++ b/include/qom/cpu.h
> @@ -227,7 +227,15 @@ struct kvm_run;
>  #define TB_JMP_CACHE_SIZE (1 << TB_JMP_CACHE_BITS)
>
>  /* Atomic insn translation TLB support. */
> +typedef struct CPUExclusiveHistory {
> +    uint16_t last_idx;           /* index of last insertion */
> +    uint16_t length;             /* history's length, it depends on smp_cpus */
> +    hwaddr *c_array;             /* history's circular array */
> +} CPUExclusiveHistory;
>  #define EXCLUSIVE_RESET_ADDR ULLONG_MAX
> +#define EXCLUSIVE_HISTORY_CPU_LEN 256
> +void cpu_exclusive_history_init(void);
> +void cpu_exclusive_history_free(void);
>
>  /**
>   * CPUState:
> diff --git a/softmmu_llsc_template.h b/softmmu_llsc_template.h
> index b4712ba..b4e7f9d 100644
> --- a/softmmu_llsc_template.h
> +++ b/softmmu_llsc_template.h
> @@ -75,6 +75,7 @@ WORD_TYPE helper_ldlink_name(CPUArchState *env, target_ulong addr,
>           * to request any flush. */
>          if (!cpu_physical_memory_is_excl(hw_addr)) {
>              cpu_physical_memory_set_excl(hw_addr);
> +            excl_history_put_addr(hw_addr);
>              CPU_FOREACH(cpu) {
>                  if (current_cpu != cpu) {
>                      tlb_flush(cpu, 1);
> diff --git a/vl.c b/vl.c
> index f043009..b22d99b 100644
> --- a/vl.c
> +++ b/vl.c
> @@ -547,6 +547,7 @@ static void res_free(void)
>  {
>      g_free(boot_splash_filedata);
>      boot_splash_filedata = NULL;
> +    cpu_exclusive_history_free();
>  }
>
>  static int default_driver_check(void *opaque, QemuOpts *opts, Error **errp)
> @@ -4322,6 +4323,8 @@ int main(int argc, char **argv, char **envp)
>
>      configure_accelerator(current_machine);
>
> +    cpu_exclusive_history_init();
> +
>      if (qtest_chrdev) {
>          qtest_init(qtest_chrdev, qtest_log, &error_fatal);
>      }


--
Alex Bennée