Re: [Qemu-devel] [PATCH] translate-all: use bitmap helpers for PageDesc's bitmap

[Paolo Bonzini <pbonzini@redhat.com>]

On 21/03/2015 07:25, Emilio G. Cota wrote:
> Note that this test
> 	if (b & ((1 << len) - 1))
> can be simplified to
> 	if (b & 1)
> , since we know that iff the first bit of a tb is set,
> all other bits from that tb are set too.

I don't think this optimization is valid, unfortunately.  It is possible
that say a tb starts at 0x12340001 and tb_invalidate_phys_page_fast is
called for the range starting at 0x12340000 and ending at 0x12340003.

In this case you need the full test that was used before.

Therefore, I am unqueuing this patch.  Sorry. :)

Paolo

> Signed-off-by: Emilio G. Cota <cota@braap.org>
> ---
>  translate-all.c | 39 +++++----------------------------------
>  1 file changed, 5 insertions(+), 34 deletions(-)
> 
> diff --git a/translate-all.c b/translate-all.c
> index 11763c6..749d0f6 100644
> --- a/translate-all.c
> +++ b/translate-all.c
> @@ -59,6 +59,7 @@
>  
>  #include "exec/cputlb.h"
>  #include "translate-all.h"
> +#include "qemu/bitmap.h"
>  #include "qemu/timer.h"
>  
>  //#define DEBUG_TB_INVALIDATE
> @@ -79,7 +80,7 @@ typedef struct PageDesc {
>      /* in order to optimize self modifying code, we count the number
>         of lookups we do to a given page to use a bitmap */
>      unsigned int code_write_count;
> -    uint8_t *code_bitmap;
> +    unsigned long *code_bitmap;
>  #if defined(CONFIG_USER_ONLY)
>      unsigned long flags;
>  #endif
> @@ -978,39 +979,12 @@ void tb_phys_invalidate(TranslationBlock *tb, tb_page_addr_t page_addr)
>      tcg_ctx.tb_ctx.tb_phys_invalidate_count++;
>  }
>  
> -static inline void set_bits(uint8_t *tab, int start, int len)
> -{
> -    int end, mask, end1;
> -
> -    end = start + len;
> -    tab += start >> 3;
> -    mask = 0xff << (start & 7);
> -    if ((start & ~7) == (end & ~7)) {
> -        if (start < end) {
> -            mask &= ~(0xff << (end & 7));
> -            *tab |= mask;
> -        }
> -    } else {
> -        *tab++ |= mask;
> -        start = (start + 8) & ~7;
> -        end1 = end & ~7;
> -        while (start < end1) {
> -            *tab++ = 0xff;
> -            start += 8;
> -        }
> -        if (start < end) {
> -            mask = ~(0xff << (end & 7));
> -            *tab |= mask;
> -        }
> -    }
> -}
> -
>  static void build_page_bitmap(PageDesc *p)
>  {
>      int n, tb_start, tb_end;
>      TranslationBlock *tb;
>  
> -    p->code_bitmap = g_malloc0(TARGET_PAGE_SIZE / 8);
> +    p->code_bitmap = bitmap_new(TARGET_PAGE_SIZE);
>  
>      tb = p->first_tb;
>      while (tb != NULL) {
> @@ -1029,7 +1003,7 @@ static void build_page_bitmap(PageDesc *p)
>              tb_start = 0;
>              tb_end = ((tb->pc + tb->size) & ~TARGET_PAGE_MASK);
>          }
> -        set_bits(p->code_bitmap, tb_start, tb_end - tb_start);
> +        bitmap_set(p->code_bitmap, tb_start, tb_end - tb_start);
>          tb = tb->page_next[n];
>      }
>  }
> @@ -1219,7 +1193,6 @@ void tb_invalidate_phys_page_range(tb_page_addr_t start, tb_page_addr_t end,
>  void tb_invalidate_phys_page_fast(tb_page_addr_t start, int len)
>  {
>      PageDesc *p;
> -    int offset, b;
>  
>  #if 0
>      if (1) {
> @@ -1235,9 +1208,7 @@ void tb_invalidate_phys_page_fast(tb_page_addr_t start, int len)
>          return;
>      }
>      if (p->code_bitmap) {
> -        offset = start & ~TARGET_PAGE_MASK;
> -        b = p->code_bitmap[offset >> 3] >> (offset & 7);
> -        if (b & ((1 << len) - 1)) {
> +        if (test_bit(start & ~TARGET_PAGE_MASK, p->code_bitmap)) {
>              goto do_invalidate;
>          }
>      } else {
>