Messages in this thread | | | Subject | Re: [PATCH 3/3] KVM: PPC: Book3S: Add support for hwrng found on some powernv systems | From | Benjamin Herrenschmidt <> | Date | Thu, 03 Oct 2013 08:02:20 +1000 |
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On Wed, 2013-10-02 at 13:02 +0300, Gleb Natapov wrote:
> Yes, I alluded to it in my email to Paul and Paolo asked also. How this > interface is disabled? Also hwrnd is MMIO in a host why guest needs to > use hypercall instead of emulating the device (in kernel or somewhere > else?).
Migration will have to be dealt with one way or another, I suppose we will indeed need a qemu fallback.
As for why hypercall instead of MMIO, well, you'd have to ask the folks who wrote the PAPR spec :-) It's specified as a hypercall and implemented as such in pHyp (PowerVM). The existing guests expect it that way.
It might have to do with the required whitening done by the hypervisor (H_RANDOM output is supposed to be clean). It also abstracts us from the underlying HW implementation which could in theory change.
> Another things is that on a host hwrnd is protected from > direct userspace access by virtue of been a device, but guest code (event > kernel mode) is userspace as far as hosts security model goes, so by > implementing this hypercall in a way that directly access hwrnd you > expose hwrnd to a userspace unconditionally. Why is this a good idea?
Why would this be a bad idea ?
Ben.
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