Re: [PATCH 2/2] target/xtensa: tidy TLB way variability logic

[Peter Maydell]

On Fri, 19 Jan 2024 at 20:47, Max Filippov <jcmvbkbc@gmail.com> wrote:
>
> Whether TLB ways 5 and 6 are variable is not a property of the TLB
> instance or a TLB entry instance, it's a property of the xtensa core
> configuration.
> Remove 'varway56' field from the xtensa_tlb structure and remove
> 'variable' field from the xtensa_tlb_entry structure. Add
> 'tlb_variable_way' array to the XtensaConfig and use it instead of
> removed fields.
>
> Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
> ---
>  target/xtensa/cpu.h          |  3 +--
>  target/xtensa/mmu_helper.c   | 38 ++++++++++--------------------------
>  target/xtensa/overlay_tool.h | 15 ++++++++++++--
>  3 files changed, 24 insertions(+), 32 deletions(-)
>
> diff --git a/target/xtensa/cpu.h b/target/xtensa/cpu.h
> index 497325466397..24d3f15ea1bf 100644
> --- a/target/xtensa/cpu.h
> +++ b/target/xtensa/cpu.h
> @@ -316,13 +316,11 @@ typedef struct xtensa_tlb_entry {
>      uint32_t paddr;
>      uint8_t asid;
>      uint8_t attr;
> -    bool variable;
>  } xtensa_tlb_entry;
>
>  typedef struct xtensa_tlb {
>      unsigned nways;
>      const unsigned way_size[10];
> -    bool varway56;
>      unsigned nrefillentries;
>  } xtensa_tlb;
>
> @@ -493,6 +491,7 @@ typedef struct XtensaConfig {
>
>      xtensa_tlb itlb;
>      xtensa_tlb dtlb;
> +    bool tlb_variable_way[16];
>
>      uint32_t mpu_align;
>      unsigned n_mpu_fg_segments;
> diff --git a/target/xtensa/mmu_helper.c b/target/xtensa/mmu_helper.c
> index d9f845e7fb6f..414c2f5ef669 100644
> --- a/target/xtensa/mmu_helper.c
> +++ b/target/xtensa/mmu_helper.c
> @@ -105,23 +105,19 @@ static uint32_t xtensa_tlb_get_addr_mask(const 
> CPUXtensaState *env,
>                                           bool dtlb, uint32_t way)
>  {
>      if (xtensa_option_enabled(env->config, XTENSA_OPTION_MMU)) {
> -        bool varway56 = dtlb ?
> -            env->config->dtlb.varway56 :
> -            env->config->itlb.varway56;
> -
>          switch (way) {
>          case 4:
>              return 0xfff00000 << get_page_size(env, dtlb, way) * 2;
>
>          case 5:
> -            if (varway56) {
> +            if (env->config->tlb_variable_way[5]) {
>                  return 0xf8000000 << get_page_size(env, dtlb, way);
>              } else {
>                  return 0xf8000000;
>              }
>
>          case 6:
> -            if (varway56) {
> +            if (env->config->tlb_variable_way[6]) {
>                  return 0xf0000000 << (1 - get_page_size(env, dtlb, way));
>              } else {
>                  return 0xf0000000;

So we now have a tlb_variable_way bool for all 16 possible
ways, but the code actually only checks it for ways 5 and 6.
Should we have an assertion somewhere that the config
doesn't try to set it on ways where it has no effect ?
Or is there actually a generic behaviour that would make
sense for eg "way 3 is variable-way" that we just don't
currently implement?

> @@ -150,11 +146,8 @@ static uint32_t get_vpn_mask(const CPUXtensaState *env, 
> bool dtlb, uint32_t way)
>          return xtensa_tlb_get_addr_mask(env, dtlb, way) << 2;
>      } else if (way <= 6) {
>          uint32_t mask = xtensa_tlb_get_addr_mask(env, dtlb, way);
> -        bool varway56 = dtlb ?
> -            env->config->dtlb.varway56 :
> -            env->config->itlb.varway56;
>
> -        if (varway56) {
> +        if (env->config->tlb_variable_way[5]) {
>              return mask << (way == 5 ? 2 : 3);
>          } else {
>              return mask << 1;

This doesn't look right -- this branch of the if-else deals
with way == 5 and way == 6, but we're only looking at
tlb_variable_way[5].

> @@ -172,10 +165,6 @@ static void split_tlb_entry_spec_way(const 
> CPUXtensaState *env, uint32_t v,
>                                       bool dtlb, uint32_t *vpn,
>                                       uint32_t wi, uint32_t *ei)
>  {
> -    bool varway56 = dtlb ?
> -        env->config->dtlb.varway56 :
> -        env->config->itlb.varway56;
> -
>      if (!dtlb) {
>          wi &= 7;
>      }
> @@ -195,7 +184,7 @@ static void split_tlb_entry_spec_way(const CPUXtensaState 
> *env, uint32_t v,
>              break;
>
>          case 5:
> -            if (varway56) {
> +            if (env->config->tlb_variable_way[5]) {
>                  uint32_t eibase = 27 + get_page_size(env, dtlb, wi);
>                  *ei = (v >> eibase) & 0x3;
>              } else {
> @@ -204,7 +193,7 @@ static void split_tlb_entry_spec_way(const CPUXtensaState 
> *env, uint32_t v,
>              break;
>
>          case 6:
> -            if (varway56) {
> +            if (env->config->tlb_variable_way[6]) {
>                  uint32_t eibase = 29 - get_page_size(env, dtlb, wi);
>                  *ei = (v >> eibase) & 0x7;
>              } else {

There's no direct code duplication, but it definitely feels like
the logic for "figure out how many bits we're dealing with" is
duplicated across these three functions.

I think it ought to be possible to have a function (or maybe two)
which take account of both the way number and tlb_get_variable_way[]
such that all of these three functions then don't need to have
a switch on the way or look at tlb_variable_way[] themselves...

thanks
-- PMM