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

[Aurelien Jarno]

On 2015-06-19 08:09, Pavel Dovgaluk wrote:
> > 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?

Yes it does. 

Aurélien

-- 
Aurelien Jarno                          GPG: 4096R/1DDD8C9B
address@hidden                 http://www.aurel32.net