Re: [Qemu-devel] [Qemu-ppc] [PATCH v2 1/2] spapr: Add support for hwrng when available

[Thomas Huth <thuth@redhat.com>]

On 08/09/15 07:03, Sam Bobroff wrote:
> On Tue, Sep 01, 2015 at 12:53:26PM +0200, Thomas Huth wrote:
>> On 01/09/15 02:38, David Gibson wrote:
>>> On Mon, Aug 31, 2015 at 08:46:01PM +0200, Thomas Huth wrote:
>>>> From: Michael Ellerman <michael@ellerman.id.au>
>>>>
>>>> Some powerpc systems have support for a hardware random number generator
>>>> (hwrng). If such a hwrng is present the host kernel can provide access
>>>> to it via the H_RANDOM hcall.
>>>>
>>>> The kernel advertises the presence of a hwrng with the KVM_CAP_PPC_HWRNG
>>>> capability. If this is detected we add the appropriate device tree bits
>>>> to advertise the presence of the hwrng to the guest kernel.
>>>>
>>>> Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
>>>> [thuth: Refreshed patch so it applies to QEMU master branch]
>>>> Signed-off-by: Thomas Huth <thuth@redhat.com>
>>>
>>> So, I'm confused by one thing.
>>>
>>> I thought new kernel handled hcalls were supposed to be disabled by
>>> default, but I don't see any calls to kvmppc_enable_hcall() to turn on
>>> H_RANDOM.
>>
>> Michael's patch was from 2013, the kvmppc_enable_hcall() stuff seems to
>> be from 2014 ... so the enablement is likely missing in this patch,
>> indeed. I didn't test the in-kernel hypercall yet, just my QEMU
>> implementation so far, that's why I did not notice this yet.
>>
>> Michael, do you want to rework your patch? Or shall I add an additional
>> enablement patch to my queue?
> 
> If I recall correctly, it's specifically not enabled: there was quite a lot of
> discussion about it when Michael posted the patches and I think the consensus
> was that it should only be enabled by QEMU, and only if the user could decide
> if it was used or not.

Can you find this discussion in a mailing list archive somewhere? The
only discussions I've found are basically these:

http://lists.nongnu.org/archive/html/qemu-devel/2013-09/msg04233.html
https://lkml.org/lkml/fancy/2013/10/1/49

... and there it was only discussed that the call should be implemented
in QEMU, too. I did not spot any discussion about making it switchable
for the user?

 Thomas