From: Laurent Dufour <ldufour@linux.vnet.ibm.com>
To: Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Michael Ellerman <mpe@ellerman.id.au>,
Simon Guo <wei.guo.simon@gmail.com>
Cc: linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
Paul Mackerras <paulus@samba.org>,
Kees Cook <keescook@chromium.org>,
Rashmica Gupta <rashmicy@gmail.com>
Subject: Re: [v4] powerpc: Export thread_struct.used_vr/used_vsr to user space
Date: Thu, 7 Jul 2016 15:40:20 +0200 [thread overview]
Message-ID: <577E5BC4.2020105@linux.vnet.ibm.com> (raw)
In-Reply-To: <1467897678.27157.45.camel@kernel.crashing.org>
On 07/07/2016 15:21, Benjamin Herrenschmidt wrote:
> On Thu, 2016-07-07 at 15:12 +0200, Laurent Dufour wrote:
>>
>> Basically, CRIU checkpoints the process register's state through the
>> ptrace API, and it restores it through a signal frame at restart time.
>> This is quite odd but that the way it works on all the CRIU's supported
>> architectures.
>>
>> Obviously everything is done from/in user space, so the sigframe
>> building too.
>> Since we can't know from user space if the thread has used or not the
>> Altivec/VSX registers, since we can't rely on the MSR bits, we always
>> dump these registers.
>
> Right, however is that an issue ? These days with glibc using V{M,S}X
> for things like memcpy I would think there is little to gain in trying
> to avoid dumping them.
>
>>> Alternately, when restoring, can you setup the sigframe with the Altivec/VSX
>>> fields populated, and the kernel will then load them, regardless of whether
>>> they were actually used or not prior to the checkpoint?
>>
>> In the case of Altivec/VSX fields, we currently force the kernel to
>> retrieve them from the signal frame by setting MSR_VEC/MSR_VSX so
>> restore_sigcontext() will copy them to the kernel thread's state.
>
> Yup, that's the way to go.
>
>> However this doesn't touch to used_vsr and used_vr which may remain at 0.
>
> That would be a kernel bug.
>
>> Most of the time this is fine, but in the case a thread which has really
>> used those registers is catching a signal just after the restore and
>> before it has touched to these registers again (and so set used_vsr/vr),
>> these registers will not be pushed in the newly built signal frame since
>> setup_sigcontext() check for used_vsr/vr before pushing the registers on
>> the stack.
>> This may be an issue in the case the thread wants to changed those
>> registers (don't ask me why :)) in the stacked signal frame from the
>> signal handler since they will not be there...
>>
>> Being able to get and set the used_vr and used_vsr thread's variables,
>> fixes this issue.
>
> I think the right fix is that if a restore_sigcontext() has the MSR bits set,
> it should set the corresponding used_* flag.
>
> Or is there a reason why that won't work ?
I got your point and I agree that most of the time now, the Altivec/VSX
registers are used by libc. In that case is there still a need for the
lazy Altivec/VSX registers dump in the signal frame ?
I'm fine with your proposal, except that every restarted process will
have the used_vr/used_vsx turned on after the restart since we can't
check if these registers were used or not at checkpoint time.
But that may be a minor point...
Cheers,
Laurent.
next prev parent reply other threads:[~2016-07-07 13:40 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-07-07 7:49 [PATCH v4] powerpc: Export thread_struct.used_vr/used_vsr to user space wei.guo.simon
2016-07-07 11:15 ` [v4] " Michael Ellerman
2016-07-07 13:12 ` Laurent Dufour
2016-07-07 13:21 ` Benjamin Herrenschmidt
2016-07-07 13:29 ` Benjamin Herrenschmidt
[not found] ` <577f7a45.568f6b0a.bd0eb.4aa8SMTPIN_ADDED_BROKEN@mx.google.com>
2016-07-11 17:39 ` Simon Guo
2016-07-15 7:15 ` Simon Guo
[not found] ` <5790aa9c.05296b0a.adf58.5901SMTPIN_ADDED_BROKEN@mx.google.com>
2016-07-25 8:52 ` Simon Guo
2016-07-07 13:40 ` Laurent Dufour [this message]
2016-07-08 5:39 ` Simon Guo
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