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Tue, 9 Mar 2021 14:46:32 +0000 (UTC) Received: from 78.163-31-62.static.virginmediabusiness.co.uk ([62.31.163.78] 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) (envelope-from ) id 1lJdd0-000ZVc-5B; Tue, 09 Mar 2021 14:46:30 +0000 Date: Tue, 09 Mar 2021 14:46:29 +0000 Message-ID: <87k0qgxu4a.wl-maz@kernel.org> From: Marc Zyngier To: Andrew Jones Subject: Re: [PATCH] KVM: arm64: Cap default IPA size to the host's own size In-Reply-To: <20210309142910.awjhb52tgj5nxs72@kamzik.brq.redhat.com> References: <20210308174643.761100-1-maz@kernel.org> <20210309132021.7vuuf73joybhlhg3@kamzik.brq.redhat.com> <87mtvcxx0z.wl-maz@kernel.org> <20210309142910.awjhb52tgj5nxs72@kamzik.brq.redhat.com> 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: 62.31.163.78 X-SA-Exim-Rcpt-To: drjones@redhat.com, kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org, kvm@vger.kernel.org, james.morse@arm.com, julien.thierry.kdev@gmail.com, suzuki.poulose@arm.com, alexandru.elisei@arm.com, kernel-team@android.com X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); SAEximRunCond expanded to false Cc: kvm@vger.kernel.org, kernel-team@android.com, linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu X-BeenThere: kvmarm@lists.cs.columbia.edu X-Mailman-Version: 2.1.14 Precedence: list List-Id: Where KVM/ARM decisions are made List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: kvmarm-bounces@lists.cs.columbia.edu Sender: kvmarm-bounces@lists.cs.columbia.edu On Tue, 09 Mar 2021 14:29:10 +0000, Andrew Jones wrote: > > On Tue, Mar 09, 2021 at 01:43:40PM +0000, Marc Zyngier wrote: > > Hi Andrew, > > > > On Tue, 09 Mar 2021 13:20:21 +0000, > > Andrew Jones wrote: > > > > > > Hi Marc, > > > > > > On Mon, Mar 08, 2021 at 05:46:43PM +0000, Marc Zyngier wrote: > > > > KVM/arm64 has forever used a 40bit default IPA space, partially > > > > due to its 32bit heritage (where the only choice is 40bit). > > > > > > > > However, there are implementations in the wild that have a *cough* > > > > much smaller *cough* IPA space, which leads to a misprogramming of > > > > VTCR_EL2, and a guest that is stuck on its first memory access > > > > if userspace dares to ask for the default IPA setting (which most > > > > VMMs do). > > > > > > > > Instead, cap the default IPA size to what the host can actually > > > > do, and spit out a one-off message on the console. The boot warning > > > > is turned into a more meaningfull message, and the new behaviour > > > > is also documented. > > > > > > > > Although this is a userspace ABI change, it doesn't really change > > > > much for userspace: > > > > > > > > - the guest couldn't run before this change, while it now has > > > > a chance to if the memory range fits the reduced IPA space > > > > > > > > - a memory slot that was accepted because it did fit the default > > > > IPA space but didn't fit the HW constraints is now properly > > > > rejected > > > > > > I'm not sure deferring the misconfiguration error until memslot > > > request time is better than just failing to create a VM. If > > > userspace doesn't use KVM_CAP_ARM_VM_IPA_SIZE to determine the > > > limit (which it hasn't been obliged to do) and it is able to > > > successfully create a VM, then it will assume up to 40-bit IPAs > > > are supported. Later, when it tries to add memslots and fails > > > it may be confused, especially if that later is much, much later > > > with memory hotplug. > > > > That's a fair point. However, no existing userspace will work on these > > systems. Is that what we want to do? I don't care much, but having > > non-usable defaults feel a bit... odd. I do spit out a warning, but I > > agree this isn't great either. > > I can send patches for QEMU, KVM selftests, and maybe even rust-vmm. > Can you point me to something about these systems I can reference > in my postings? Or I can just reference this mail thread. The system of choice to see this is an Apple M1 box. Not supported in mainline yet, but things are progressing pretty quickly. > > > > > > > The other thing that's left doing is to convince userspace to > > > > actually use the IPA space setting instead of relying on the > > > > antiquated default. > > > > > > Failing to create any VM which hasn't selected a valid IPA limit > > > should be pretty convincing :-) > > > > I'll make sure to redirect the reports your way! :D > > What's the current error message when this occurs? Is it good enough, or > should we improve it to help provide people hints? Please don't change > it to "Invalid IPA limit, please mail Andrew Jones" :-) Well, that's part of the problem. Currently, you don't get a message, and the guest faults on its first memory access forever (level 0 translation fault), as the VTCR_EL2.T0SZ value is bogus. I can change this patch to reject 40bit IPA when requested as a default with something saying "Userspace using unsupported default IPA limit, upgrade your VMM". Now, there is another nit[1] which I just found with my kvmtool setup that computes the optimal IPA space for a given VM. And that one is even more problematic... Thanks, M. [1] https://lore.kernel.org/r/87lfawxv40.wl-maz@kernel.org -- Without deviation from the norm, progress is not possible. _______________________________________________ kvmarm mailing list kvmarm@lists.cs.columbia.edu https://lists.cs.columbia.edu/mailman/listinfo/kvmarm