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Received: from crackle.ozlabs.ibm.com (ppp121-45-219-3.bras1.cbr2.internode.on.net [121.45.219.3]) by mail.messagingengine.com (Postfix) with ESMTPA id B51C410310; Wed, 20 Feb 2019 06:18:54 -0500 (EST) Message-ID: <275066ee653036b139a548de70965015062f4c48.camel@russell.cc> Subject: Re: [PATCH] powerpc/powernv/idle: Restore IAMR after idle From: Russell Currey To: Akshay Adiga , Nicholas Piggin Date: Wed, 20 Feb 2019 22:18:51 +1100 In-Reply-To: <20190220060353.GA21952@aks.ibm> References: <20190206062837.26917-1-ruscur@russell.cc> <1549515373.con208q1rq.astroid@bobo.none> <87o97njdjg.fsf@concordia.ellerman.id.au> <1550549702.xfczazszdw.astroid@bobo.none> <20190220060353.GA21952@aks.ibm> Content-Type: text/plain; charset="UTF-8" User-Agent: Evolution 3.30.5 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linuxppc-dev@lists.ozlabs.org Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On Wed, 2019-02-20 at 11:34 +0530, Akshay Adiga wrote: > On Tue, Feb 19, 2019 at 02:21:04PM +1000, Nicholas Piggin wrote: > > Michael Ellerman's on February 8, 2019 11:04 am: > > > Nicholas Piggin writes: > > > > Russell Currey's on February 6, 2019 4:28 pm: > > > > > Without restoring the IAMR after idle, execution prevention > > > > > on POWER9 > > > > > with Radix MMU is overwritten and the kernel can freely > > > > > execute userspace without > > > > > faulting. > > > > > > > > > > This is necessary when returning from any stop state that > > > > > modifies user > > > > > state, as well as hypervisor state. > > > > > > > > > > To test how this fails without this patch, load the lkdtm > > > > > driver and > > > > > do the following: > > > > > > > > > > echo EXEC_USERSPACE > /sys/kernel/debug/provoke- > > > > > crash/DIRECT > > > > > > > > > > which won't fault, then boot the kernel with powersave=off, > > > > > where it > > > > > will fault. Applying this patch will fix this. > > > > > > > > > > Fixes: 3b10d0095a1e ("powerpc/mm/radix: Prevent kernel > > > > > execution of user > > > > > space") > > > > > Cc: > > > > > Signed-off-by: Russell Currey > > > > > > > > Good catch and debugging. This really should be a quirk, we > > > > don't want > > > > to have to restore this thing on a thread switch. > > > > > > I'm not sure I follow. We don't context switch it on Radix, but > > > we do > > > on hash if pkeys are enabled. > > > > Badly worded, I mean a hardware quirk. It should follow thread > > switches. Still, avoiding it for the no-loss case is better than > > nothing. We can just revisit it as an optimization if future > > hardware does not require the restore. > > Apparently, the POWER9 Processor User’s Manual v2.0 documents that > IAMR can be lost, and that is not just the end. > > Pasting excerpt from "Section 23.5.9.2 State Loss and > Restoration,Page 309" > > On the POWER9 core, the only state that can be lost for > Stop levels less than four, when PSSCR[ESL] = ‘1’ are the > following SPRs: CR, FPSCR, VSCR, XER, DSCR, AMR, IAMR, UAMOR, > AMOR, DAWR, DAWRX. > > My observation is that AMOR is being used in kernel as of today > and AMOR is also lost (recreated in similar scenarios where > IAMR is lost). > I can add AMOR to this patch (or you can send a patch, either way).