Re: [Qemu-devel] [PATCH v2 0/3] Fix exceptions handling for MIPS and i386

[Paolo Bonzini]

On 18/06/2015 11:42, Aurelien Jarno wrote:
>> > QEMU could just always compute and store the restore_state information.
>> >  TCG needs to help filling it in (a new TCG opcode?), but it should be 
>> > easy.
> Yes, that was another approach I have in mind (I called it exception
> table in my other mail),

Okay, understood.  My idea was more like always generating the gen_op_*
arrays.

> but it requires a tiny more work than just
> saving the CPU state all the time. The problem is that the state
> information we want to save are varying for target to target. Going
> through a TCG opcode means we can use the liveness analysis pass to save
> the minimum amount of data.

I mentioned a TCG opcode because the target PC is not available inside
the translator.  So the translator could pepper the TCG instruction
stream with things like

     checkpoint  $target_pc, $target_cc_op, $0

TCG can then use them to fill in an array stored inside the
TranslationBlock, together with the host PC.  Since the gen_opc_pc,
gen_opc_instr_start, gen_opc_icount arrays are inside tcg_ctx, it may be
a good idea to store the checkpoint information compressed in a byte
array (e.g. as a series of ULEB128 values---the host and target PCs can
even be stored as deltas from the last value).

As a first step, gen_intermediate_code_pc and tcg_gen_code_search_pc can
then be merged into a single target-independent function that
uncompresses the byte array up to the required host PC into tcg_ctx.
Later you can optimize them to remove the tcg_ctx arrays altogether.

So the patches could be something like this:

1) SPARC: put the jump target information directly in gen_opc_* without
using gen_opc_jump_pc (not trivial)

2) a few targets: instead of gen_opc_* arrays, use a new generic member
of tcg_ctx (similar to how csbase is used generically), e.g.
tcg_ctx.gen_opc_target1[] and tcg_ctx.gen_opc_target2[].

3) all targets: always fill in tcg_ctx.gen_*, even if search_pc is false

4) TCG: add support for a checkpoint operation, make it fill in
tcg_ctx.gen_*

5) all targets: change explicit filling of tcg_ctx.gen_* to use the
checkpoint operation

6) TCG/translate-all: convert gen_intermediate_code_pc as outlined above

> That said I would like to push further the idea of always saving the CPU
> state a bit more to see if we can keep the same performances. There are
> still improvements to do, by removing more code on the core side (like
> finding the call to tb_finc_pc which is now useless), or on the target
> side by checking/improving helper flags. We might save the CPU state too
> often if a helper doesn't declare it doesn't touch globals.

True, on the other hand there are a lot of helpers to audit...

Paolo