Re: [Qemu-devel] [PATCH] cpu: implementing victim TLB for QEMU system emulated TLB

[Richard Henderson <rth@twiddle.net>]

On 01/22/2014 06:48 AM, Xin Tong wrote:
> +#define TLB_XOR_SWAP(X, Y) do {*X = *X ^ *Y; *Y = *X ^ *Y; *X = *X ^
> *Y;}while(0);

First, your patch is line wrapped.  You really really really need to follow the
directions Peter gave you.

Second, using xor to swap values is a cute assembler trick, but it has no place
in high-level programming.  Look at the generated assembly and you'll find way
more memory accesses than necessary.

> +void swap_tlb(CPUTLBEntry *te, CPUTLBEntry *se, hwaddr *iote, hwaddr *iose)

This function could probably stand to be inline, so that we produce better code
for softmmu_template.h.

> +        for (k = 0;k < CPU_VTLB_SIZE; k++) {

Watch your spacing.  Did the patch pass checkpatch.pl?

>          for (mmu_idx = 0; mmu_idx < NB_MMU_MODES; mmu_idx++) {
>              unsigned int i;
> -
>              for (i = 0; i < CPU_TLB_SIZE; i++) {

Don't randomly change whitespace.

> +    /* do not discard the translation in te, evict it into a random
> victim tlb */
> +    unsigned vidx = rand() % CPU_VTLB_SIZE;

Don't use rand.  That's a huge heavy-weight function.  Treating the victim
table as a circular buffer would surely be quicker.  Using a LRU algorithm
might do better, but could also be overkill.

>              do_unaligned_access(env, addr, READ_ACCESS_TYPE, mmu_idx, retaddr);
>          }
>  #endif
> -        tlb_fill(env, addr, READ_ACCESS_TYPE, mmu_idx, retaddr);
> +        /* we are about to do a page table walk. our last hope is the
> victim tlb.
> +         * try to refill from the victim tlb before walking the page table. */
> +        int vidx, vhit = false;

We're supposed to be c89 compliant.  No declarations in the middle of the
block.  Also, you can avoid the vhit variable entirely with

> +        for(vidx = 0;vidx < CPU_VTLB_SIZE; ++vidx) {

  for (vidx = CPU_VTLB_SIZE - 1; vidx >= 0; --vidx) {
      ...
  }
  if (vidx < 0) {
      tlb_fill(...);
  }


r~