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Re: [Qemu-devel] [RFC 02/10] use a different translation block list for

From: Frederic Konrad
Subject: Re: [Qemu-devel] [RFC 02/10] use a different translation block list for each cpu.
Date: Mon, 02 Feb 2015 09:39:25 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.5.0

On 29/01/2015 16:24, Peter Maydell wrote:
On 16 January 2015 at 17:19,  <address@hidden> wrote:
From: KONRAD Frederic <address@hidden>

We need a different TranslationBlock list for each core in case of multithread

Signed-off-by: KONRAD Frederic <address@hidden>
  translate-all.c | 40 ++++++++++++++++++++++------------------
  1 file changed, 22 insertions(+), 18 deletions(-)

diff --git a/translate-all.c b/translate-all.c
index 8fa4378..0e11c70 100644
--- a/translate-all.c
+++ b/translate-all.c
@@ -72,10 +72,11 @@

+#define MAX_CPUS 256

  typedef struct PageDesc {
      /* list of TBs intersecting this ram page */
-    TranslationBlock *first_tb;
+    TranslationBlock *first_tb[MAX_CPUS];
Do we really need to know this for every CPU, or just for
the one that's using this PageDesc? I am assuming we're going to make
the l1_map be per-CPU.

Do we have any clue of which cpu is using this PageDesc?
We did this like that because it is quite simple.

      /* 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;
@@ -750,7 +751,7 @@ static inline void invalidate_page_bitmap(PageDesc *p)
  /* Set to NULL all the 'first_tb' fields in all PageDescs. */
  static void page_flush_tb_1(int level, void **lp)
-    int i;
+    int i, j;

      if (*lp == NULL) {
@@ -759,7 +760,9 @@ static void page_flush_tb_1(int level, void **lp)
          PageDesc *pd = *lp;

          for (i = 0; i < V_L2_SIZE; ++i) {
-            pd[i].first_tb = NULL;
+            for (j = 0; j < MAX_CPUS; j++) {
+                pd[i].first_tb[j] = NULL;
+            }
              invalidate_page_bitmap(pd + i);
      } else {
@@ -937,12 +940,12 @@ void tb_phys_invalidate(TranslationBlock *tb, 
tb_page_addr_t page_addr)
      /* remove the TB from the page list */
      if (tb->page_addr[0] != page_addr) {
          p = page_find(tb->page_addr[0] >> TARGET_PAGE_BITS);
-        tb_page_remove(&p->first_tb, tb);
+        tb_page_remove(&p->first_tb[current_cpu->cpu_index], tb);
Anything using current_cpu in this code is hugely suspect.
For instance cpu_restore_state() takes a CPUState pointer and
calls this function -- either it should be acting on just that
CPU (which might not be the current one) or on all CPUs. In
any case implicitly working on current_cpu here is wrong.

Probably we need to look at the public-facing functions here
and decide which should have "operate on all CPUs" semantics
and which should have "operate on the CPU passed as a parameter"
and which "operate on the implicit current CPU".

Ok so the idea would be to have eg a cpu mask parameter to know which cpu to
invalidate/restore etc etc?
Or just pointer and invalidate all if NULL?


-- PMM

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