On Mon, May 18, 2020 at 06:20:55PM +0100, Catalin Marinas wrote: > On Mon, May 18, 2020 at 12:31:03PM +0100, Will Deacon wrote: > > On Mon, May 18, 2020 at 12:26:30PM +0100, Vladimir Murzin wrote: > > > On 5/15/20 6:16 PM, Catalin Marinas wrote: > > > > For performance analysis it may be desirable to disable MTE altogether > > > > via an early param. Introduce arm64.mte_disable and, if true, filter out > > > > the sanitised ID_AA64PFR1_EL1.MTE field to avoid exposing the HWCAP to > > > > user. > > > > > > > > Signed-off-by: Catalin Marinas <catalin.marinas@xxxxxxx> > > > > Cc: Will Deacon <will@xxxxxxxxxx> > > > > --- > > > > > > > > Notes: > > > > New in v4. > > > > > > > > Documentation/admin-guide/kernel-parameters.txt | 4 ++++ > > > > arch/arm64/kernel/cpufeature.c | 11 +++++++++++ > > > > 2 files changed, 15 insertions(+) > > > > > > > > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt > > > > index f2a93c8679e8..7436e7462b85 100644 > > > > --- a/Documentation/admin-guide/kernel-parameters.txt > > > > +++ b/Documentation/admin-guide/kernel-parameters.txt > > > > @@ -373,6 +373,10 @@ > > > > arcrimi= [HW,NET] ARCnet - "RIM I" (entirely mem-mapped) cards > > > > Format: <io>,<irq>,<nodeID> > > > > > > > > + arm64.mte_disable= > > > > + [ARM64] Disable Linux support for the Memory > > > > + Tagging Extension (both user and in-kernel). > > > > + > > > > > > Should it really to take parameter (on/off/true/false)? It may lead to expectation > > > that arm64.mte_disable=false should enable MT and, yes, double negatives make it > > > look ugly, so if we do need parameter, can it be arm64.mte=on/off/true/false? > > > > I don't think "performance analysis" is a good justification for this > > parameter tbh. We don't tend to add these options for other architectural > > features, and I don't see why MTE is any different in this regard. > > There is an expectation of performance impact with MTE enabled, > especially if it's running in synchronous mode. For the in-kernel MTE, > we could add a parameter which sets sync vs async at boot time rather > than a big disable knob. It won't affect user space however. > > The other 'justification' is if your hardware has weird unexpected > behaviour but I'd like this handled via errata workarounds. > > I'll let the people who asked for this to chip in ;). I agree with you > that we rarely add these (and I rejected a similar option a few weeks > ago on the AMU patchset). We've been looking into other ways this on/off behavior could be achieved. The "arm,armv8.5-memtag" DT flag already provides what we want - meaning that this flag could be removed if the system did not support MTE. I did see your remark on "arm64: mte: Check the DT memory nodes for MTE support" questioning whether it was the right approach - is this still the case? --Patrick -- Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
