Re: [Qemu-devel] [PATCH 3/4] accel/tcg: Add cluster number to TCG TB hash

[Aleksandar Markovic]

On Tuesday, January 8, 2019, Peter Maydell <address@hidden> wrote:

> Include the cluster number in the hash we use to look
> up TBs. This is important because a TB that is valid
> for one cluster at a given physical address and set
> of CPU flags is not necessarily valid for another:
> the two clusters may have different views of physical
> memory, or may have different CPU features (eg FPU
> present or absent).
>
>
Hi, Peter.

I do understand the definition of cluster_index in the sense of this
series. However, it looks to me that the term "cluster" is generally
overused in areas where we work. This may lead to some confusion for future
developers, and let me suggest some other name, like "tcg_cluster_index" or
"tcg_group_id", or "translation_group_id". Admitedly, they all sound ugly
to me too. But having the name that would clearly separate this id from too
generic "cluster_index" IMHO would save lots of time during potential
related future development.

(Needled to say that,  for example, we in MIPS, for multi-core sustems,
group cores in clusters, that actually do not have anything to do with
clusters in TCG sense...)

Sincerely,

Aleksandar



> We put the cluster number in the high 8 bits of the
> TB cflags. This gives us up to 256 clusters, which should
> be enough for anybody. If we ever need more, or need
> more bits in cflags for other purposes, we could make
> tb_hash_func() take more data (and expand qemu_xxhash7()
> to qemu_xxhash8()).
>
> Signed-off-by: Peter Maydell <address@hidden>
> ---
>  include/exec/exec-all.h   | 4 +++-
>  accel/tcg/cpu-exec.c      | 4 ++++
>  accel/tcg/translate-all.c | 3 +++
>  3 files changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/include/exec/exec-all.h b/include/exec/exec-all.h
> index 815e5b1e838..aa7b81aaf01 100644
> --- a/include/exec/exec-all.h
> +++ b/include/exec/exec-all.h
> @@ -351,9 +351,11 @@ struct TranslationBlock {
>  #define CF_USE_ICOUNT  0x00020000
>  #define CF_INVALID     0x00040000 /* TB is stale. Set with @jmp_lock held
> */
>  #define CF_PARALLEL    0x00080000 /* Generate code for a parallel context
> */
> +#define CF_CLUSTER_MASK 0xff000000 /* Top 8 bits are cluster ID */
> +#define CF_CLUSTER_SHIFT 24
>  /* cflags' mask for hashing/comparison */
>  #define CF_HASH_MASK   \
> -    (CF_COUNT_MASK | CF_LAST_IO | CF_USE_ICOUNT | CF_PARALLEL)
> +    (CF_COUNT_MASK | CF_LAST_IO | CF_USE_ICOUNT | CF_PARALLEL |
> CF_CLUSTER_MASK)
>
>      /* Per-vCPU dynamic tracing state used to generate this TB */
>      uint32_t trace_vcpu_dstate;
> diff --git a/accel/tcg/cpu-exec.c b/accel/tcg/cpu-exec.c
> index 870027d4359..e578a1a3aee 100644
> --- a/accel/tcg/cpu-exec.c
> +++ b/accel/tcg/cpu-exec.c
> @@ -336,6 +336,10 @@ TranslationBlock *tb_htable_lookup(CPUState *cpu,
> target_ulong pc,
>          return NULL;
>      }
>      desc.phys_page1 = phys_pc & TARGET_PAGE_MASK;
> +
> +    cf_mask &= ~CF_CLUSTER_MASK;
> +    cf_mask |= cpu->cluster_index << CF_CLUSTER_SHIFT;
> +
>      h = tb_hash_func(phys_pc, pc, flags, cf_mask, *cpu->trace_dstate);
>      return qht_lookup_custom(&tb_ctx.htable, &desc, h, tb_lookup_cmp);
>  }
> diff --git a/accel/tcg/translate-all.c b/accel/tcg/translate-all.c
> index 639f0b27287..ba27f5acc8c 100644
> --- a/accel/tcg/translate-all.c
> +++ b/accel/tcg/translate-all.c
> @@ -1692,6 +1692,9 @@ TranslationBlock *tb_gen_code(CPUState *cpu,
>          cflags |= CF_NOCACHE | 1;
>      }
>
> +    cflags &= ~CF_CLUSTER_MASK;
> +    cflags |= cpu->cluster_index << CF_CLUSTER_SHIFT;
> +
>   buffer_overflow:
>      tb = tb_alloc(pc);
>      if (unlikely(!tb)) {
> --
> 2.19.2
>
>
>