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[209.132.180.67]) by mx.google.com with ESMTP id k1-v6si18038428pgj.522.2018.09.18.08.16.41; Tue, 18 Sep 2018 08:17:29 -0700 (PDT) Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67; Authentication-Results: mx.google.com; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729900AbeIRUtd (ORCPT + 99 others); Tue, 18 Sep 2018 16:49:33 -0400 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70]:46420 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727756AbeIRUtc (ORCPT ); Tue, 18 Sep 2018 16:49:32 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 3BE4E7A9; Tue, 18 Sep 2018 08:16:31 -0700 (PDT) Received: from [10.4.13.23] (en101.emea.arm.com [10.4.13.23]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id CF2DE3F5BD; Tue, 18 Sep 2018 08:16:28 -0700 (PDT) From: Suzuki K Poulose Subject: Re: [PATCH v5 18/18] kvm: arm64: Allow tuning the physical address size for VM To: Peter Maydell Cc: arm-mail-list , kvmarm@lists.cs.columbia.edu, kvm-devel , Marc Zyngier , Christoffer Dall , Eric Auger , Paolo Bonzini , =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= , Will Deacon , Catalin Marinas , James Morse , Dave P Martin , julien.grall@arm.com, lkml - Kernel Mailing List References: <20180917104144.19188-1-suzuki.poulose@arm.com> <20180917104144.19188-19-suzuki.poulose@arm.com> Message-ID: <218b64d5-741d-97ef-8aae-bc0a2ac3e56c@arm.com> Date: Tue, 18 Sep 2018 16:16:27 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=us-ascii; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Peter, On 18/09/2018 02:55, Peter Maydell wrote: > On 17 September 2018 at 11:41, Suzuki K Poulose wrote: >> --- a/Documentation/virtual/kvm/api.txt >> +++ b/Documentation/virtual/kvm/api.txt >> @@ -122,6 +122,14 @@ the default trap & emulate implementation (which changes the virtual >> memory layout to fit in user mode), check KVM_CAP_MIPS_VZ and use the >> flag KVM_VM_MIPS_VZ. >> >> +To configure the physical address space size for a VM (IPA size) on arm64, >> +check KVM_CAP_ARM_VM_PHYS_SHIFT (which returns the maximum limit for the >> +IPA shift) and use KVM_VM_TYPE_ARM_PHYS_SHIFT(PHYS_SHIFT). Bits[7-0] of the >> +machine type has been reserved for specifying the PHYS_SHIFT. >> +The supported range is [32...IPA_LIMIT], where IPA_LIMIT could be >> +identified by checking KVM_CAP_ARM_VM_PHYS_SHIFT. For backward compatibility >> +a value of 0 selects 40bits. >> + > > Given this as the API documentation, I don't think I could figure out > what I as a userspace user of it need to do without looking at the > kernel code. Could I ask you to expand it so that it is a bit less > terse and a bit more detailed? (For instance, what is a PHYS_SHIFT > and why do I have to specify it rather than just telling the kernel > I want a 48 bit guest address space?) Thanks for the feedback. I acknowledge that the documentation is not quite clear for a userspace user. How about: "To configure the physical address space size for a VM (IPA size) on arm64, check KVM_CAP_ARM_VM_IPA_SIZE and use KVM_VM_TYPE_ARM_IPA_SIZE(IPA_Bits) as the argument to KVM_CREATE_VM, where IPA_Bits is the maximum width of any physical address used by the VM. The IPA_Bits is encoded in Bits[7-0] of the machine type, and must be one of { 0, 32, ... , Host_IPA_Limit }, where : 1) IPA_Bits = 0 implies 40bits IPA (for backward compatibility) 2) Host_IPA_Limit is the maximum limit for IPA_Bits on the host, which is dependent on the CPU capability and the host kernel configuration. This can be detected by checking the extension KVM_CAP_ARM_VM_IPA_SIZE " note: I have renamed the KVM_CAP_ARM_VM_PHYS_SHIFT => KVM_CAP_ARM_VM_IPA_SIZE above and can update the code, if the latter is better. Suzuki