On Tue, Sep 08, 2015 at 03:03:16PM +1000, 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 <mich...@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 <mich...@ellerman.id.au> > > >> [thuth: Refreshed patch so it applies to QEMU master branch] > > >> Signed-off-by: Thomas Huth <th...@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. > > What if we set up another backend that just enables the hcall in KVM?
I think that's basically the right approach.
It can't quite be a "backend" as such, since the in-kernel hcall can
only supply H_RANDOM; it can't supply random for other purposes like
virtio-rng, which the general qemu rng backends can.
So I'd suggest two options controlling H_RANDOM:
usekvm : boolean [default true]
Whether to enable the in-kernel implementation if
available
backend : ref to rng backend object [no default]
Backend to use if in-kernel implementation is
unavailable or disabled.
At this point rather than just implementing them as discrete machine
options, I suspect it will be more maintainable to split out the
h-random implementation as a pseudo-device with its own qdev and so
forth. We already do similarly for the RTAS time of day functions
(spapr-rtc).
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
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