Re: [uml-devel] Strange behaviour of hwrng

[Blaisorblade <blaisorblade@yahoo.it>]

On Friday 21 October 2005 20:24, Gerd Stolpmann wrote:
> Hi list,
>
> I recently tested the hwrng driver. In principal, it works, but it
> sometimes eats up all host CPU time. In particular, I can see that the
> UML system does a (blocking) read on /dev/hwrng,

Could you please elaborate on that? Who is doing the read, the rng tools? 
Also, are they doing only a single read (in which case, it's we the ones who 
are looping and we're broken) or they are astonished by our -EAGAIN and keep 
trying (in which case, they are broken)?

Actually, from looking at the code (arch/um/drivers/random.c:rng_dev_read()) 
it appears that we _do_ loop ourselves, if the read is blocking. Not sure how 
to handle that.

We could return maybe -EIO, possibly when a rate limit is exceeded (not 
trivial to do, though - I must learn using timers first).
> and that the host 
> system loops while reading from /dev/random which almost always returns
> -EAGAIN. (Found that out with strace, in the hope the output is
> correct.)

Well, if the host hasn't enough entropy, it's reasonable for it to return 
-EAGAIN.

And we should do the same (the loop is actually executed by the UML code, 
right?).

However, probably the tools inside UML don't expect a lot to get -EAGAIN from 
an hardware generator. So possibly they are not ready to handle that well.

> Nevertheless rngd seems to work, because when killing with SIGUSR1 it
> shows some reasonable statistics.

> My test setup is as follows:

> - Host system: 2.6.12.6 with SKAS3 v8.2 patch
> - UML system: 2.6.14-rc5, in SKAS3 mode.
>   Using rngd from Debian Sarge.
>   TLS is disabled.

> Let me know if you need more information.

> Gerd

-- 
Inform me of my mistakes, so I can keep imitating Homer Simpson's "Doh!".
Paolo Giarrusso, aka Blaisorblade (Skype ID "PaoloGiarrusso", ICQ 215621894)
http://www.user-mode-linux.org/~blaisorblade

	

	
		
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