Message-ID: <4199C603.4000701@fujitsu-siemens.com>
From: Bodo Stroesser <bstroesser@fujitsu-siemens.com>
MIME-Version: 1.0
Subject: Re: [uml-devel] [Patch 1/1] uml: fix uml-use-sysemu-for-tt.patch
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Date: Tue, 16 Nov 2004 10:18:59 +0100
To: Blaisorblade <blaisorblade_spam@yahoo.it>
Cc: user-mode-linux-devel@lists.sourceforge.net, Jeff Dike <jdike@addtoit.com>

Blaisorblade wrote:
> On Monday 15 November 2004 20:04, Bodo Stroesser wrote:
> 
>>Blaisorblade wrote:
>>
>>>On Friday 12 November 2004 15:10, Bodo Stroesser wrote:
>>>
>>>>From: Bodo Stroesser <bstroesser@fujitsu-siemens.com>
>>>>
>>>>The patch needs some small corrections:
>>>>1) local_using_sysemu must be sampled *before* the next
>>>>   ptrace(PTRACE_SYSEMU/SYSCALL) and must stay the same until
>>>>do_syscall() has been done. Currently it is sampled before do_syscall()
>>>>and is used after this for ptrace(PTRACE_SYSEMU/SYSCALL). Even if no
>>>>problem is visible to the UML user, a single syscall could be executed
>>>>on the host when switching on sysemu. The result of this then is
>>>>overwritten by the syscall execution in UML.
> 
> 
>>>>   Since the first event the tracer has to handle is not a syscall, it's
>>>>   enough to initialize local_using_sysemu to 0;
> 
> 
>>>Sorry, what happens if the first signal it gets is a SIGTRAP, and so
>>>local_using_sysemu is not yet set? If this is impossible, please add a
>>>comment in the code for this. However, it seems that it can get to the
>>>SIGTRAP case with tracing == 1. When beginning the procedure, it is 0,
>>>but it can be changed with the value from is_tracing(task). I've not
>>>checked if that is zeroed on process creation (i.e. by do_fork() calling
>>>copy_thread()), but just note that in the code.
> 
> 
>>OK: Let's summarize:
>>1) tracer() is started exactly once.
>>2) The first this it does, is starting the first ptraced-process via
>>clone(). 3) Then it waits until the new process stops.
>>4) Since the process will run start_kernel() in kernel space, it is resumed
>>    with PTRACE_CONT.
>>Thus, before having any syscall interception, the process has to stop
>>itself with a SIGUSR1, giving the tracer an OP_TRACE_OP request. After this
>>local_using_sysemu will be set and the process will be resumed with
>>PTRACE_SYSCALL or PTRACE_SYSEMU.
> 
> 
>>>>2) Even if the host never *does* a syscall in SYSEMU, we have to write
>>>>the syscall number with -1, to not have the host doing syscall
>>>>restarting. This would happen only with an invalid syscall number equal
>>>>to one of the -ERESTART values.
>>>>
>>>>
>>>>   But to be perfect ...
> 
> 
>>>Yes, but shouldn't this be handled on the host? Restarting a syscall
>>>which has never been done does not seem something that SYSEMU should
>>>allow... I don't want anybody to need going through the code and checking
>>>that this is safe.
> 
> 
>>Yes. It should. So I exactly added this to the "advanced sysemu". But
>>despite this UML should work on an older sysemu host, too. So I add this
>>now, and with the SYSEMU_SINGLESTEP-patches a skip of writing -1 is
>>inserted, if the new sysemu is in use.
> 
> Hmm - not yet had time to get near that (I'm busy), however if 
> SYSEMU_SINGLESTEP fixes this API inconsistency, the old SYSEMU API will 
> probably not be accepted by mainline developers, not it should be sent by us 
> - remember that we want to try merging SYSEMU sooner than SKAS4.
> 
> No single UML (except some ones using the sysemu incremental version, which we 
> don't want to support) is unable to run without SYSEMU, so dropping that 
> support should be ok.
> 
> Also, this new mode must wait for now - get some time to test it, I'll do the 
> same when I've time. Currently I have too few time.
No problem. I already tested a lot. But it should be tested *very* well, before
creating a new SKAS3 version from it.
I'm planning this to be the first version that is supported by s390-host!


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