From: Christoffer Dall <christoffer.dall@linaro.org>
To: Peter Maydell <peter.maydell@linaro.org>
Cc: Marc Zyngier <marc.zyngier@arm.com>,
Claudio Fontana <claudio.fontana@huawei.com>,
Jan Kiszka <jan.kiszka@web.de>,
"kvmarm@lists.cs.columbia.edu" <kvmarm@lists.cs.columbia.edu>,
arm-mail-list <linux-arm-kernel@lists.infradead.org>,
kvm-devel <kvm@vger.kernel.org>,
Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [RFC PATCH] KVM: arm/arm64: Don't let userspace update CNTVOFF once guest is running
Date: Thu, 9 Jul 2015 14:24:06 +0200 [thread overview]
Message-ID: <20150709122406.GN13530@cbox> (raw)
In-Reply-To: <CAFEAcA94qRKs7TV4MmrhD4O0+cE4SYPb4iKydQQbSzdrzKMC+w@mail.gmail.com>
On Thu, Jul 09, 2015 at 01:07:24PM +0100, Peter Maydell wrote:
> On 9 July 2015 at 13:05, Christoffer Dall <christoffer.dall@linaro.org> wrote:
> > As I understand it, the problem is that if we ever run a VCPU after
> > reading the value, and write back the value afterwards, you potentially
> > make time go backwards and get inconsistent views of time from different
> > VCPUs because they may have read the time before/after updating the
> > CNTVOFF.
>
> Right, but I think if QEMU does that it's a bug (and more to
> the point I don't entirely understand why we would do that
> yet, even given that we don't have a distinction between
> "registers to sync always" and "registers to sync only on
> reset"...)
>
I think we have evidence that it does that, but we don't know why/how.
-Christoffer
WARNING: multiple messages have this Message-ID (diff)
From: christoffer.dall@linaro.org (Christoffer Dall)
To: linux-arm-kernel@lists.infradead.org
Subject: [RFC PATCH] KVM: arm/arm64: Don't let userspace update CNTVOFF once guest is running
Date: Thu, 9 Jul 2015 14:24:06 +0200 [thread overview]
Message-ID: <20150709122406.GN13530@cbox> (raw)
In-Reply-To: <CAFEAcA94qRKs7TV4MmrhD4O0+cE4SYPb4iKydQQbSzdrzKMC+w@mail.gmail.com>
On Thu, Jul 09, 2015 at 01:07:24PM +0100, Peter Maydell wrote:
> On 9 July 2015 at 13:05, Christoffer Dall <christoffer.dall@linaro.org> wrote:
> > As I understand it, the problem is that if we ever run a VCPU after
> > reading the value, and write back the value afterwards, you potentially
> > make time go backwards and get inconsistent views of time from different
> > VCPUs because they may have read the time before/after updating the
> > CNTVOFF.
>
> Right, but I think if QEMU does that it's a bug (and more to
> the point I don't entirely understand why we would do that
> yet, even given that we don't have a distinction between
> "registers to sync always" and "registers to sync only on
> reset"...)
>
I think we have evidence that it does that, but we don't know why/how.
-Christoffer
next prev parent reply other threads:[~2015-07-09 12:24 UTC|newest]
Thread overview: 46+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-06-24 14:54 [RFC PATCH] KVM: arm/arm64: Don't let userspace update CNTVOFF once guest is running Marc Zyngier
2015-06-24 14:54 ` Marc Zyngier
2015-06-25 8:04 ` Christoffer Dall
2015-06-25 8:04 ` Christoffer Dall
2015-06-25 8:48 ` Marc Zyngier
2015-06-25 8:48 ` Marc Zyngier
2015-06-25 8:59 ` Claudio Fontana
2015-06-25 8:59 ` Claudio Fontana
2015-06-25 9:10 ` Peter Maydell
2015-06-25 9:10 ` Peter Maydell
2015-06-25 9:25 ` Claudio Fontana
2015-06-25 9:25 ` Claudio Fontana
2015-06-26 4:49 ` Jan Kiszka
2015-06-26 4:49 ` Jan Kiszka
2015-06-29 17:20 ` Claudio Fontana
2015-06-29 17:20 ` Claudio Fontana
2015-06-29 17:37 ` Peter Maydell
2015-06-29 17:37 ` Peter Maydell
2015-07-08 15:56 ` Marc Zyngier
2015-07-08 15:56 ` Marc Zyngier
2015-07-08 16:06 ` Peter Maydell
2015-07-08 16:06 ` Peter Maydell
2015-07-08 16:37 ` Marc Zyngier
2015-07-08 16:37 ` Marc Zyngier
2015-07-08 19:13 ` Peter Maydell
2015-07-08 19:13 ` Peter Maydell
2015-07-09 10:22 ` Christoffer Dall
2015-07-09 10:22 ` Christoffer Dall
2015-07-09 10:38 ` Peter Maydell
2015-07-09 10:38 ` Peter Maydell
2015-07-09 12:05 ` Christoffer Dall
2015-07-09 12:05 ` Christoffer Dall
2015-07-09 12:07 ` Peter Maydell
2015-07-09 12:07 ` Peter Maydell
2015-07-09 12:24 ` Christoffer Dall [this message]
2015-07-09 12:24 ` Christoffer Dall
2015-07-09 14:17 ` Christoffer Dall
2015-07-09 14:17 ` Christoffer Dall
2015-07-09 14:26 ` Peter Maydell
2015-07-09 14:26 ` Peter Maydell
2015-07-09 16:06 ` Christoffer Dall
2015-07-09 16:06 ` Christoffer Dall
2015-07-09 10:40 ` Jan Kiszka
2015-07-09 10:40 ` Jan Kiszka
2015-07-09 12:08 ` Christoffer Dall
2015-07-09 12:08 ` Christoffer Dall
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