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From: Blue Swirl <blauwirbel@gmail.com>
Date: Sat, 11 Dec 2010 14:44:44 +0000
Message-ID: <AANLkTi=e-BTWSHmAAKKMyBCtQfMvGwPP-JewuToibuk8@mail.gmail.com>
Subject: Re: [Qemu-devel] TCG flow vs dyngen
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To: Stefano Bonifazi <stefboombastic@gmail.com>
Cc: qemu-devel@nongnu.org

On Sat, Dec 11, 2010 at 2:32 PM, Stefano Bonifazi
<stefboombastic@gmail.com> wrote:
> -----Original Message-----
> From: Blue Swirl [mailto:blauwirbel@gmail.com]
> Sent: sabato 11 dicembre 2010 14:12
> To: Stefano Bonifazi
> Cc: qemu-devel@nongnu.org
> Subject: Re: [Qemu-devel] TCG flow vs dyngen
>
>
>>There's a large buffer for generated code, allocated in exec.c. This is f=
illed with host code by TCG, when full it is flushed. The CPU execution loo=
p >generates new TBs when needed, otherwise the old code can be executed.
>
>>TCG also uses intermediate ops but those are used only once during transl=
ation.
>
> So if I understand well the flow is the following:
>
> 1) the CPU execution loop at runtime takes a new TB from the target code
> 2) I guess some hash function is computed on this TB for getting a key fo=
r searching into the buffer of generated code that probably should store th=
e binary as a map key-binary
> 3) if the search is successful the binary is given to the translator(how?=
 You said no copy involved) and we return to point 1) otherwise:

1-3) Please see tb_find_fast() and its caller in cpu-exec.c. Only
pointer passing is involved.

> 4) the target specific translator generates TCG uops from the TB
> 5) TCG uses uops for generating =C2=A0host binary code
> 6) this new binary code is cached by TGC if there is enough storage place
>
> Is that all correct?

4-5) OK.
6) If there is no space, all previously generated code is thrown away.

>
> Where does the execution of host binary take place in the previous list o=
f events? =C2=A0Between point 5) and 6) ?
> After 6) ? In what QEMU source code file/function does the final executio=
n of host binary take place?
>
> In the previous list of events, when does the translator try to chain the=
 current TB with previous ones? =C2=A0Before TCG generates the binary in or=
der to feed it with "linked micro code"?

All of this happens in cpu-exec.c:581 to 618.