From mboxrd@z Thu Jan 1 00:00:00 1970 From: mark.rutland@arm.com (Mark Rutland) Date: Wed, 14 Sep 2016 11:36:46 +0100 Subject: [PATCH v3 0/7] arm64: Privileged Access Never using TTBR0_EL1 switching In-Reply-To: References: <1473788797-10879-1-git-send-email-catalin.marinas@arm.com> <20160914102659.GC14330@leverpostej> Message-ID: <20160914103632.GD14330@leverpostej> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Wed, Sep 14, 2016 at 11:30:05AM +0100, Ard Biesheuvel wrote: > On 14 September 2016 at 11:27, Mark Rutland wrote: > > On Wed, Sep 14, 2016 at 11:13:33AM +0100, Ard Biesheuvel wrote: > >> On 13 September 2016 at 18:46, Catalin Marinas wrote: > >> > This is the third version of the arm64 PAN emulation using TTBR0_EL1 > >> > switching. > > > >> Given that every __get_user() call now incurs the PAN switch overhead, > >> I wonder if it would be worth it to stash the real TTBR0_EL1 value in, > >> e.g., TPIDRRO_EL0 rather than load it from memory each time. We'd have > >> to reload the real value of TPIDRRO_EL0 at kernel exit every time, but > >> only for compat tasks, and not nearly as often, obviously. > > > > FWIW, my plan for vmap'd stacks involves clobbering TPIDRRO_EL0 early > > upon kernel entry to reliably detect/handle stack overflow (as we need > > to free up GPR to detect overflow, and we need to detect that before we > > try to store to the stack). > > > > For non-compat tasks we must restore zero, so either way we'll end up > > with a load (to determine compat-ness or to load the precise value). > > Are you saying that with vmapped stacks, we'll end up clobbering it > (and thus restoring it) anyway when entering the kernel, and so we > could use it for free afterwards while running in the kernel, > potentially for the real value of TTBR0_EL1? Yes, assuming that we end up following my current plan for how to implement that. Thanks, Mark.