From: Paul Mackerras <paulus@samba.org>
To: Gleb Natapov <gleb@redhat.com>
Cc: tytso@mit.edu, kvm@vger.kernel.org, linuxppc-dev@ozlabs.org,
Alexander Graf <agraf@suse.de>,
kvm-ppc@vger.kernel.org, linux-kernel@vger.kernel.org,
herbert@gondor.hengli.com.au, mpm@selenic.com,
Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH 3/3] KVM: PPC: Book3S: Add support for hwrng found on some powernv systems
Date: Thu, 3 Oct 2013 20:06:30 +1000 [thread overview]
Message-ID: <20131003100630.GA27821@iris.ozlabs.ibm.com> (raw)
In-Reply-To: <20131003054803.GU17294@redhat.com>
On Thu, Oct 03, 2013 at 08:48:03AM +0300, Gleb Natapov wrote:
> On Thu, Oct 03, 2013 at 08:45:42AM +1000, Paul Mackerras wrote:
> > On Wed, Oct 02, 2013 at 04:36:05PM +0200, Alexander Graf wrote:
> > >
> > > On 02.10.2013, at 16:33, Paolo Bonzini wrote:
> > >
> > > > Il 02/10/2013 16:08, Alexander Graf ha scritto:
> > > >>> 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.
> > > >
> > > > With Michael's earlier patch in this series, the hwrng is accessible by
> > > > host userspace via /dev/hwrng, no?
> > >
> > > Yes, but there's not token from user space that gets passed into the kernel to check whether access is ok or not. So while QEMU may not have permission to open /dev/hwrng it could spawn a guest that opens it, drains all entropy out of it and thus stall other processes which try to fetch entropy, no?
> >
> > Even if you drain all entropy out of it, wait 64 microseconds and it
> > will be full again. :) Basically it produces 64 bits every
> > microsecond and puts that in a 64 entry x 64-bit FIFO buffer, which is
> > what is read by the MMIO. So there is no danger of stalling other
> > processes for any significant amount of time.
> >
> Even if user crates 100s guests each one of which reads hwrng in a loop?
Well, you can't actually have more guests running than there are cores
in a system. POWER7+ has one RNG per chip and 8 cores per chip, each
of which can run 4 threads (which have to be in the same guest).
Michael's code uses the RNG on the same chip. Worst case therefore is
32 threads accessing the same RNG, so a given thread might have to
wait up to 32 microseconds for its data.
Paul.
next prev parent reply other threads:[~2013-10-03 10:06 UTC|newest]
Thread overview: 46+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-09-26 6:31 [PATCH 1/3] powerpc: Implement arch_get_random_long/int() for powernv Michael Ellerman
2013-09-26 6:31 ` [PATCH 2/3] hwrng: Add a driver for the hwrng found in power7+ systems Michael Ellerman
2013-09-26 8:01 ` Benjamin Herrenschmidt
2013-10-01 8:25 ` Michael Ellerman
2013-09-26 6:31 ` [PATCH 3/3] KVM: PPC: Book3S: Add support for hwrng found on some powernv systems Michael Ellerman
2013-09-26 9:06 ` Paolo Bonzini
2013-10-01 8:34 ` Michael Ellerman
2013-10-01 8:39 ` Gleb Natapov
2013-10-01 9:23 ` Paul Mackerras
2013-10-01 9:57 ` Gleb Natapov
2013-10-01 10:00 ` Alexander Graf
2013-10-01 9:38 ` Benjamin Herrenschmidt
2013-10-01 11:19 ` Paolo Bonzini
2013-10-01 21:44 ` Benjamin Herrenschmidt
2013-10-02 8:38 ` Paolo Bonzini
2013-10-02 5:09 ` Paul Mackerras
2013-10-02 8:46 ` Paolo Bonzini
2013-10-02 9:06 ` Benjamin Herrenschmidt
2013-10-02 9:11 ` Alexander Graf
2013-10-02 9:50 ` Alexander Graf
2013-10-02 10:02 ` Gleb Natapov
2013-10-02 13:57 ` Michael Ellerman
2013-10-02 14:08 ` Alexander Graf
2013-10-02 14:33 ` Paolo Bonzini
2013-10-02 14:36 ` Alexander Graf
2013-10-02 14:38 ` Paolo Bonzini
2013-10-02 22:45 ` Paul Mackerras
2013-10-03 5:48 ` Gleb Natapov
2013-10-03 10:06 ` Paul Mackerras [this message]
2013-10-03 12:08 ` Gleb Natapov
2013-10-02 14:37 ` Gleb Natapov
2013-10-02 22:21 ` Benjamin Herrenschmidt
2013-10-03 6:08 ` Gleb Natapov
2013-10-02 22:13 ` Benjamin Herrenschmidt
2013-10-02 14:10 ` Gleb Natapov
2013-10-02 22:15 ` Benjamin Herrenschmidt
2013-10-02 22:02 ` Benjamin Herrenschmidt
2013-10-03 5:43 ` Gleb Natapov
2013-10-03 7:22 ` Benjamin Herrenschmidt
2013-10-02 22:07 ` Benjamin Herrenschmidt
2013-10-03 6:28 ` Gleb Natapov
2013-10-02 21:58 ` Benjamin Herrenschmidt
2013-10-01 9:58 ` Paolo Bonzini
2013-09-27 14:15 ` Anshuman Khandual
2013-10-01 8:36 ` Michael Ellerman
2013-09-26 7:58 ` [PATCH 1/3] powerpc: Implement arch_get_random_long/int() for powernv Benjamin Herrenschmidt
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