Re: [Qemu-devel] [PATCH v6 1/3] cputlb: do not evict empty entries to the vtlb

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

Emilio G. Cota <cota@braap.org> writes:

> Currently we evict an entry to the victim TLB when it doesn't match
> the current address. But it could be that there's no match because
> the current entry is empty (i.e. all -1's, for instance via tlb_flush).
> Do not evict the entry to the vtlb in that case.
>
> This change will help us keep track of the TLB's use rate, which
> we'll use to implement a policy for dynamic TLB sizing.
>
> Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
> Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
> Signed-off-by: Emilio G. Cota <cota@braap.org>
> ---
>  include/exec/cpu-all.h | 9 +++++++++
>  accel/tcg/cputlb.c     | 2 +-
>  2 files changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/include/exec/cpu-all.h b/include/exec/cpu-all.h
> index 117d2fbbca..e21140049b 100644
> --- a/include/exec/cpu-all.h
> +++ b/include/exec/cpu-all.h
> @@ -362,6 +362,15 @@ static inline bool tlb_hit(target_ulong tlb_addr, target_ulong addr)
>      return tlb_hit_page(tlb_addr, addr & TARGET_PAGE_MASK);
>  }
>
> +/**
> + * tlb_entry_is_empty - return true if the entry is not in use
> + * @te: pointer to CPUTLBEntry
> + */
> +static inline bool tlb_entry_is_empty(const CPUTLBEntry *te)
> +{
> +    return te->addr_read == -1 && te->addr_write == -1 && te->addr_code == -1;
> +}
> +

This breaks on a --disable-tcg build. I think the whole block needs to
be in a:

  #if !defined(CONFIG_USER_ONLY) && defined(CONFIG_TCG)

now.

>  void dump_exec_info(FILE *f, fprintf_function cpu_fprintf);
>  void dump_opcount_info(FILE *f, fprintf_function cpu_fprintf);
>  #endif /* !CONFIG_USER_ONLY */
> diff --git a/accel/tcg/cputlb.c b/accel/tcg/cputlb.c
> index af6bd8ccf9..5dc97212a9 100644
> --- a/accel/tcg/cputlb.c
> +++ b/accel/tcg/cputlb.c
> @@ -591,7 +591,7 @@ void tlb_set_page_with_attrs(CPUState *cpu, target_ulong vaddr,
>       * Only evict the old entry to the victim tlb if it's for a
>       * different page; otherwise just overwrite the stale data.
>       */
> -    if (!tlb_hit_page_anyprot(te, vaddr_page)) {
> +    if (!tlb_hit_page_anyprot(te, vaddr_page) && !tlb_entry_is_empty(te)) {
>          unsigned vidx = env->tlb_d[mmu_idx].vindex++ % CPU_VTLB_SIZE;
>          CPUTLBEntry *tv = &env->tlb_v_table[mmu_idx][vidx];


--
Alex Bennée