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Re: [Qemu-devel] Re: [4799] Add instruction counter.
From: |
Jan Kiszka |
Subject: |
Re: [Qemu-devel] Re: [4799] Add instruction counter. |
Date: |
Sun, 29 Jun 2008 15:54:52 +0200 |
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Mozilla/5.0 (X11; U; Linux i686 (x86_64); de; rv:1.8.1.12) Gecko/20080226 SUSE/2.0.0.12-1.1 Thunderbird/2.0.0.12 Mnenhy/0.7.5.666 |
Paul Brook wrote:
>> On the first glance this function looked like it could serve as an
>> alternative to SSTEP_INTERNAL and provide the required roll-back on
>> watchpoint hit. But looking closer I realized that icount_decr is only
>> maintained if use_icount is set.
>
> I'm fairly sure limiting the length of the TB and actual instruction counting
> are largely independent. IIUC you only need the former.
But to calculate the former, you need the latter again. I wonder if it
wouldn't be more efficient and flexible to specify a terminating PC
instead of an instruction count. Wouldn't that make cpu_io_recompile
independent of icount_decr and, thus, use_icount?
>
>> I do not yet get why you were forced to go a different path for
>> cpu_io_recompile, ie. rebuilding and (re-executing?) the whole TB up to
>> the instruction that caused the IO access instead of just regenerating a
>> single-insn TB for that purpose. Is it more efficient?
>
> Generating a single insn IO TB is a good idea for resolving the current
> fault.
> This is what the comment at the end of cpu_io_recompile is referring to.
>
> Regenerating a truncated version of the original version of the TB is
> important for subsequent execution of that block. MMIO accesses occur
> frequently in loops when the guest is checking status bits or accessing a
> FIFO. Recompiling the TB means that subsequent accesses complete with
> minimal overhead. If we didn't recompile then every access would incur a
> (very expensive) trap+unwind+singlestep.
>
> The type of access can't be determined statically (it's a property of the
> address being accesses, not the instruction itself). However I'd expect that
> most accesses always access wither RAM or MMIO spaces in practice, so
> recompiling when we see an IO access is a reasonable compromise.
OK, understood.
>
>> But if use_icount is off by default, I guess this doesn't come for free
>> either...
>
> See above. cpu_io_recompile is used to get precise delivery of interrupts.
> This is required for but not dependent on having deterministic timing (i.e.
> use_icount).
Watchpoints, specifically guest-injected ones, require deterministic
exception delivery as well. So I would like to reuse existing
infrastructure that already solved a similar problem.
Jan
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Re: [Qemu-devel] [4799] Add instruction counter., C.W. Betts, 2008/06/29