Re: [Qemu-devel] [Qemu-ppc] [RFC PATCH] ppc/spapr_hcall: Implement H_RANDOM hypercall

[Thomas Huth <thuth@redhat.com>]

On 06/08/15 13:26, Laurent Vivier wrote:
> Hi,
> 
> As "/dev/random" can be blocking perhaps it is possible/better to use
> the QEMU rng backend instead ?

Actually, I tried to use the rng backends first ... but they turned out
to be horrible for being used in a synchronous call like I need here:
You've got to install a callback function which is called at a "random"
point in time when some data becomes available - and it is even called
with less bytes than you requested! (i.e. I requested 8 bytes, but the
callback got only called with 6 bytes). So to use it, I'd have to
introduce a ring buffer or something similar to query enough random data
in advance - and when it runs empty, the H_RANDOM hypercall would need
to block again anyway.

So instead of introducing a hard-to-understand-and-maintain "monster
wrapper" around the rng backend functions, I think the short and easy
understandable qemu_random() implementation in this patch here is the
better choice.

And looking at https://patchwork.ozlabs.org/patch/497062/ I think it
also should not hurt too much that qemu_random() might be slow here
since the real hardware number generator is apparently also not very fast.

 Thomas


> On 06/08/2015 10:23, Thomas Huth wrote:
>> The spapr firmware SLOF likely needs a way to get random numbers
>> soon. Instead of re-inventing the wheel there, we could simply
>> use the H_RANDOM hypercall to get the random numbers from QEMU.
>> For this the H_RANDOM hypercall needs to be implemented first, of
>> course.
>> So this patch introduces an OS-specifc helper function called
>> qemu_random() to get "good" random numbers (from /dev/random on
>> POSIX systems and via CryptGenRandom() on Windows), and then uses
>> this helper functions to provide random data to the guest via the
>> H_RANDOM hypercall.
>>
>> Signed-off-by: Thomas Huth <thuth@redhat.com>
>> ---
>>  Note: Since I do not have access to the SPAPR spec, I figured out
>>  the calling conventions of the hypercall by looking at the way
>>  the Linux kernel is using this call to retriev random data. So
>>  there might be some bugs or missing pieces here.
>>
>>  Also note that the Windows version of qemu_random() is
>>  compile-tested only (with a MinGW cross-compiler), since I do not
>>  have access to a Windows  system right now.
>>
>>  hw/ppc/spapr_hcall.c   | 12 ++++++++++++
>>  include/hw/ppc/spapr.h |  1 +
>>  include/qemu/osdep.h   | 11 +++++++++++
>>  util/oslib-posix.c     | 16 ++++++++++++++++
>>  util/oslib-win32.c     | 17 +++++++++++++++++
>>  5 files changed, 57 insertions(+)