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Re: [Qemu-discuss] Singlestepping & Target assembly instructions

From: Peter Maydell
Subject: Re: [Qemu-discuss] Singlestepping & Target assembly instructions
Date: Tue, 15 Sep 2015 22:23:48 +0100

On 15 September 2015 at 21:51, Scott Dattalo <address@hidden> wrote:
> On 9/15/2015 12:30 PM, Peter Maydell wrote:
>> On 15 September 2015 at 19:38, Scott Dattalo <address@hidden>
>> wrote:
>>> Question: how can you tell QEmu to create an instruction trace?
>> You can't, to a first approximation. QEMU's debug logging is
>> aimed more at telling you what QEMU has done than providing
>> detailed information on what the guest program is doing.
> Thanks Peter. I also supply the '-singlestepping' option and was under the
> impression this would break the TB chains, or more specifically, flush the
> TB cache.

No, singlestep does neither of those two things. All it does
is force the front end to put exactly one guest instruction
into each generated TB.

> I think there is a fork produce by Cedric Vincent:
> https://github.com/cedric-vincent/qemu/tree/master/tcg/plugins
> The idea is to allow the TCI to be instrumented with scripts. The QEmu
> source in github does not contain this code.

Yes. The problem is that there's a gap between "good enough
for my purposes" and "good enough to be able to put into
master" which is very wide and hard to bridge. (For tracing,
you need to have all of:
 * doesn't reduce maintainability of the code
 * works on all targets and hosts (ideally)
 * doesn't reduce performance if you're not using it
 * doesn't need to be specially enabled at compile time
for instance.)

I'd like to see better tracing but it needs somebody who
has the time and determination to get working code into
production QEMU.

-- PMM

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