[Interest] [Development] QRandomGenerator and boot times

Sami Nurmenniemi sami.nurmenniemi at qt.io
Mon Sep 18 10:44:31 CEST 2017

On 18/09/2017 7:59 pm, "Christian Gagneraud" <chgans at gmail.com 
<mailto:chgans at gmail.com>> wrote:
>     On 18/09/2017 7:34 pm, "Sami Nurmenniemi" <sami.nurmenniemi at qt.io
>     <mailto:sami.nurmenniemi at qt.io>> wrote:
>         On 15.09.2017 18:21, Thiago Macieira wrote:
>             On Friday, 15 September 2017 00:31:36 PDT Sami Nurmenniemi
>             wrote:
>                 I think we'll just have to accept blocking for the
>                 devices without
>                 hwrng. I don't know if we really support any such
>                 devices. If we do and
>                 boot time is essential for those, we'll have to figure
>                 out some way
>                 (probably saving entropy over reboot).
>             And that would be a non-Qt job. I wasn't worried about Qt
>             on real devices
>             because I don't expect it to be run early enough to
>             matter.  On VMs, that's
>             another story, and if you don't trust your
>             undercloud-provided /dev/hwrng, why
>             are you using that cloud? (Though I'll say there's an
>             Intel team working on
>             figuring out how a VM can get trust from the actual
>             hardware, skipping the
>             hypervisor trust)
>         We have made some safety critical customer demos where fast
>         boot time of Qt framework is essential. It was demoing boot
>         time of 1.2s for a Qt application to start after powering on
>         the device. I suppose that demo (and real safety critical use
>         cases) is going to have problems with the QRandomGenerator
>         even with hwrng in use.
>     So basically, you have artificially "optimised" boot time by
>     ignoring entropy management and now you're blaming
>     QRandomGenerator. You can do one or the other, not both.
>     Me too I can make a device boot in less than a second, but then
>     don't be surprised if some features are missing/buggy....
>     This sound very much like a beginner mistake, where boot time
>     takes over functionality. Every one can boot an unfunctional
>     device in less than a second. This is nothing new.
The point of mentioning the 1.2s demo was an answer to the statement "I 
wasn't worried about Qt on real devices because I don't expect it to be 
run early enough to matter." Maybe the demo application (being the only 
user space process) needs to make the connection between hwrng + random 
pool or preserve entropy over reboots.

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