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Thu, 17 Jun 2021 13:53:46 +0000 Received: from mail.kernel.org ([198.145.29.99]) by bombadil.infradead.org with esmtps (Exim 4.94.2 #2 (Red Hat Linux)) id 1ltsSk-00AZqX-BC for linux-arm-kernel@lists.infradead.org; Thu, 17 Jun 2021 13:53:43 +0000 Received: from disco-boy.misterjones.org (disco-boy.misterjones.org [51.254.78.96]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 11EFD611CE; Thu, 17 Jun 2021 13:53:42 +0000 (UTC) Received: from sofa.misterjones.org ([185.219.108.64] helo=why.misterjones.org) by disco-boy.misterjones.org with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.94.2) (envelope-from ) id 1ltsSi-008BoA-28; Thu, 17 Jun 2021 14:53:40 +0100 Date: Thu, 17 Jun 2021 14:53:39 +0100 Message-ID: <87fsxgd2cc.wl-maz@kernel.org> From: Marc Zyngier To: Steven Price Cc: Catalin Marinas , Will Deacon , James Morse , Julien Thierry , Suzuki K Poulose , kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Dave Martin , Mark Rutland , Thomas Gleixner , qemu-devel@nongnu.org, Juan Quintela , "Dr. David Alan Gilbert" , Richard Henderson , Peter Maydell , Haibo Xu , Andrew Jones Subject: Re: [PATCH v15 0/7] MTE support for KVM guest In-Reply-To: References: <20210614090525.4338-1-steven.price@arm.com> <20210617121322.GC6314@arm.com> <87im2cd443.wl-maz@kernel.org> User-Agent: Wanderlust/2.15.9 (Almost Unreal) SEMI-EPG/1.14.7 (Harue) FLIM-LB/1.14.9 (=?UTF-8?B?R29qxY0=?=) APEL-LB/10.8 EasyPG/1.0.0 Emacs/27.1 (x86_64-pc-linux-gnu) MULE/6.0 (HANACHIRUSATO) MIME-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue") X-SA-Exim-Connect-IP: 185.219.108.64 X-SA-Exim-Rcpt-To: steven.price@arm.com, catalin.marinas@arm.com, will@kernel.org, james.morse@arm.com, julien.thierry.kdev@gmail.com, suzuki.poulose@arm.com, kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Dave.Martin@arm.com, mark.rutland@arm.com, tglx@linutronix.de, qemu-devel@nongnu.org, quintela@redhat.com, dgilbert@redhat.com, richard.henderson@linaro.org, peter.maydell@linaro.org, Haibo.Xu@arm.com, drjones@redhat.com X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); SAEximRunCond expanded to false X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20210617_065342_470342_23FE78DD X-CRM114-Status: GOOD ( 44.65 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Thu, 17 Jun 2021 14:24:25 +0100, Steven Price wrote: > > On 17/06/2021 14:15, Marc Zyngier wrote: > > On Thu, 17 Jun 2021 13:13:22 +0100, > > Catalin Marinas wrote: > >> > >> On Mon, Jun 14, 2021 at 10:05:18AM +0100, Steven Price wrote: > >>> I realise there are still open questions[1] around the performance of > >>> this series (the 'big lock', tag_sync_lock, introduced in the first > >>> patch). But there should be no impact on non-MTE workloads and until we > >>> get real MTE-enabled hardware it's hard to know whether there is a need > >>> for something more sophisticated or not. Peter Collingbourne's patch[3] > >>> to clear the tags at page allocation time should hide more of the impact > >>> for non-VM cases. So the remaining concern is around VM startup which > >>> could be effectively serialised through the lock. > >> [...] > >>> [1]: https://lore.kernel.org/r/874ke7z3ng.wl-maz%40kernel.org > >> > >> Start-up, VM resume, migration could be affected by this lock, basically > >> any time you fault a page into the guest. As you said, for now it should > >> be fine as long as the hardware doesn't support MTE or qemu doesn't > >> enable MTE in guests. But the problem won't go away. > > > > Indeed. And I find it odd to say "it's not a problem, we don't have > > any HW available". By this token, why should we merge this work the > > first place, or any of the MTE work that has gone into the kernel over > > the past years? > > > >> We have a partial solution with an array of locks to mitigate against > >> this but there's still the question of whether we should actually bother > >> for something that's unlikely to happen in practice: MAP_SHARED memory > >> in guests (ignoring the stage 1 case for now). > >> > >> If MAP_SHARED in guests is not a realistic use-case, we have the vma in > >> user_mem_abort() and if the VM_SHARED flag is set together with MTE > >> enabled for guests, we can reject the mapping. > > > > That's a reasonable approach. I wonder whether we could do that right > > at the point where the memslot is associated with the VM, like this: > > > > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c > > index a36a2e3082d8..ebd3b3224386 100644 > > --- a/arch/arm64/kvm/mmu.c > > +++ b/arch/arm64/kvm/mmu.c > > @@ -1376,6 +1376,9 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm, > > if (!vma) > > break; > > > > + if (kvm_has_mte(kvm) && vma->vm_flags & VM_SHARED) > > + return -EINVAL; > > + > > /* > > * Take the intersection of this VMA with the memory region > > */ > > > > which takes the problem out of the fault path altogether? We document > > the restriction and move on. With that, we can use a non-locking > > version of mte_sync_page_tags(). > > Does this deal with the case where the VMAs are changed after the > memslot is created? While we can do the check here to give the VMM a > heads-up if it gets it wrong, I think we also need it in > user_mem_abort() to deal with a VMM which mmap()s over the VA of the > memslot. Or am I missing something? No, you're right. I wish the memslot API wasn't so lax... Anyway, even a VMA flag check in user_mem_abort() will be cheaper than this new BKL. > But if everyone is happy with the restriction (just for KVM) of not > allowing MTE+VM_SHARED then that sounds like a good way forward. Definitely works for me. M. -- Without deviation from the norm, progress is not possible. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel