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Re: [Qemu-devel] [PATCH 3/5] tcg: always keep jump target and tb->jmp_ne

From: Sergey Fedorov
Subject: Re: [Qemu-devel] [PATCH 3/5] tcg: always keep jump target and tb->jmp_next consistent
Date: Fri, 18 Mar 2016 13:32:11 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0

On 17/03/16 23:45, Sergey Fedorov wrote:
On 17/03/16 22:31, Paolo Bonzini wrote:

On 17/03/2016 18:57, Richard Henderson wrote:
@@ -951,18 +959,10 @@ static inline void tb_jmp_remove(TranslationBlock *tb, int n)
          /* now we can suppress tb(n) from the list */
          *ptb = tb->jmp_next[n];
-        tb->jmp_next[n] = NULL;
+        tb_reset_jump(tb, n);
What's the motivation here?  This implies an extra cache flush.
Where were we resetting the jump previously?  Or is this a bug
in that we *weren't* resetting the jump previously?
Indeed I think this patch can be removed if it has a performance effect
on machines that require icache invalidation.  If it doesn't, it would
be just a small code simplification.

In fact, tb_jmp_remove() is only supposed to remove the TB from a list of all TB's jumping to the same TB which is n-th jump destination of the given TB. This function is only called in tb_phys_invalidate() for the TB being invalidated. Thus we don't have to patch that TB anymore. We don't even have to do "tb->jmp_next[n] = NULL" here.

I'll drop the patch from the series in v2 then.

Kind regards,

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