From mboxrd@z Thu Jan 1 00:00:00 1970 From: christoffer.dall@linaro.org (Christoffer Dall) Date: Wed, 10 Feb 2016 13:44:59 +0100 Subject: [PATCH 6/8] KVM: arm/arm64: timer: Add active state caching In-Reply-To: <1454931622-14902-7-git-send-email-marc.zyngier@arm.com> References: <1454931622-14902-1-git-send-email-marc.zyngier@arm.com> <1454931622-14902-7-git-send-email-marc.zyngier@arm.com> Message-ID: <20160210124459.GM5171@cbox> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Mon, Feb 08, 2016 at 11:40:20AM +0000, Marc Zyngier wrote: > Programming the active state in the (re)distributor can be an > expensive operation so it makes some sense to try and reduce > the number of accesses as much as possible. So far, we > program the active state on each VM entry, but there is some > opportunity to do less. > > An obvious solution is to cache the active state in memory, > and only program it in the HW when conditions change. But > because the HW can also change things under our feet (the active > state can transition from 1 to 0 when the guest does an EOI), > some precautions have to be taken, which amount to only caching > an "inactive" state, and always programing it otherwise. > > With this in place, we observe a reduction of around 700 cycles > on a 2GHz GICv2 platform for a NULL hypercall. > > Signed-off-by: Marc Zyngier Reviewed-by: Christoffer Dall