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

[Sam Bobroff]

On Tue, Sep 08, 2015 at 07:38:12AM +0200, Thomas Huth wrote:
> 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 <address@hidden>
> >>>>
> >>>> 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 <address@hidden>
> >>>> [thuth: Refreshed patch so it applies to QEMU master branch]
> >>>> Signed-off-by: Thomas Huth <address@hidden>
> >>>
> >>> 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?

Sorry that part wasn't very well worded: I was trying to say that QEMU's
configuration should be able to choose how H_RANDOM handled, rather than having
it automatically choose based on what was available in KVM and that's what
we're considering now anyway. (See David's comment elsewhere in this thread.)

Sam.