Subject: Re: [PATCH 3/3] KVM: PPC: Book3S: Add support for hwrng found on some powernv systems
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From: Alexander Graf <agraf@suse.de>
In-Reply-To: <1380722275.12149.28.camel@concordia>
Date: Wed, 2 Oct 2013 16:08:47 +0200
Cc: Gleb Natapov <gleb@redhat.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paolo Bonzini <pbonzini@redhat.com>, Paul Mackerras <paulus@samba.org>,
        linux-kernel@vger.kernel.org, mpm@selenic.com,
        herbert@gondor.hengli.com.au, linuxppc-dev@ozlabs.org,
        kvm@vger.kernel.org, kvm-ppc@vger.kernel.org, tytso@mit.edu
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References: <5243F933.7000907@redhat.com> <20131001083426.GB27484@concordia> <20131001083908.GA17294@redhat.com> <1380620338.645.22.camel@pasglop> <524AAFAA.3010801@redhat.com> <20131002050940.GA25363@drongo> <524BDD73.3020106@redhat.com> <1380704789.645.57.camel@pasglop> <668E4650-BC22-4CBF-A282-E7875DF29DB6@suse.de> <3CBF5732-E7EE-4C96-8132-6D7B77270DAF@suse.de> <20131002100224.GF17294@redhat.com> <1380722275.12149.28.camel@concordia>
To: Michael Ellerman <michael@ellerman.id.au>
Sender: linux-kernel-owner@vger.kernel.org
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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

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