Re: [PATCH 3/3] KVM: PPC: Book3S: Add support for hwrng found on some powernv systems

Alexander Graf
2013-10-02T14:08:56Z

On 02.10.2013, at 15:57, Michael Ellerman wrote:

> On Wed, 2013-10-02 at 13:02 +0300, Gleb Natapov wrote:
>> On Wed, Oct 02, 2013 at 11:50:50AM +0200, Alexander Graf wrote:
>>>
>>> On 02.10.2013, at 11:11, Alexander Graf wrote:
>>>
>>> So how do you solve live migration between a kernel that has this patch and one that doesn't?
>>>
>> Yes, I alluded to it in my email to Paul and Paolo asked also. How this
>> interface is disabled?
>
> Yes that is a valid point.
>
> We can't disable the interface at runtime, the guest detects its
> presence at boot.
>
> What will happen is the hcall will come through to QEMU, which will
> reject it with H_FUNCTION (~= ENOSYS).
>
> The current pseries-rng driver does not handle that case well, which is
> exactly why I sent patches to fix it recently.
>
> The only other option would be to feed it with /dev/random.
>
>> Also hwrnd is MMIO in a host why guest needs to
>> use hypercall instead of emulating the device (in kernel or somewhere
>> else?).
>
> Because PAPR is a platform specification and it specifies that the
> interface is a hypervisor call. We can't just decide we want to do it
> differently.
>
>> 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?
>
> I'm not sure I follow you.
>
> The hwrng is accessible by host userspace via /dev/mem.

A guest should live on the same permission level as a user space application. If you run QEMU as UID 1000 without access to /dev/mem, why should the guest suddenly be able to directly access a memory location (MMIO) it couldn't access directly through a normal user space interface.

It's basically a layering violation.

Alex