From mboxrd@z Thu Jan 1 00:00:00 1970 From: christoffer.dall@linaro.org (Christoffer Dall) Date: Thu, 9 Jul 2015 14:24:06 +0200 Subject: [RFC PATCH] KVM: arm/arm64: Don't let userspace update CNTVOFF once guest is running In-Reply-To: References: <55917E5B.3030401@huawei.com> <559D483A.40507@arm.com> <559D51C5.3070600@arm.com> <20150709102201.GH13530@cbox> <20150709120531.GL13530@cbox> Message-ID: <20150709122406.GN13530@cbox> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Thu, Jul 09, 2015 at 01:07:24PM +0100, Peter Maydell wrote: > On 9 July 2015 at 13:05, Christoffer Dall 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