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

[Pavel Dovgaluk]

> From: Aurelien Jarno [mailto:address@hidden
> On 2015-06-18 12:02, Paolo Bonzini wrote:
> >
> > 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).
> 
> Either as deltas to the last value or as delta from the start of the
> TB. What I am worried about is the size of the checkpoint information,
> even if we do some compression, we might have one per guest instruction.
> I have implemented a naive version of that without compression, storing
> the checkpoint data at the end of the generated code, and it's about 30%
> of the size of the TB for MIPS. It's probably smaller on architectures
> storing only the PC. Also it's size is quite variable. That's why it's
> probably not a good idea to store it directly in the TranslationBlock.
> I don't like storing it directly in the generated code either,
> especially given this part is supposed to be executable.
> 
> > 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's sounds like a plan when I have more time ;-)

Doesn't this approach still require my fixes to work correctly?

Pavel Dovgalyuk