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[209.132.180.67]) by mx.google.com with ESMTP id k1-v6si4635378pgh.65.2018.07.09.05.32.01; Mon, 09 Jul 2018 05:32:40 -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 S932984AbeGIM3t (ORCPT + 99 others); Mon, 9 Jul 2018 08:29:49 -0400 Received: from foss.arm.com ([217.140.101.70]:58436 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932595AbeGIM3s (ORCPT ); Mon, 9 Jul 2018 08:29:48 -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 9F3F27A9; Mon, 9 Jul 2018 05:29:47 -0700 (PDT) Received: from [10.1.206.75] (usa-sjc-imap-foss1.foss.arm.com [10.72.51.249]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 42D013F5AD; Mon, 9 Jul 2018 05:29:44 -0700 (PDT) Subject: Re: [PATCH v3 15/20] kvm: arm/arm64: Allow tuning the physical address size for VM To: Dave Martin , Suzuki K Poulose Cc: Will Deacon , cdall@kernel.org, kvm@vger.kernel.org, catalin.marinas@arm.com, punit.agrawal@arm.com, linux-kernel@vger.kernel.org, qemu-devel@nongnu.org, Paolo Bonzini , kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org References: <1530270944-11351-1-git-send-email-suzuki.poulose@arm.com> <1530270944-11351-16-git-send-email-suzuki.poulose@arm.com> <20180704155104.GN4828@arm.com> <12d1832a-1a13-7dd4-662b-addf58400789@arm.com> <9f1af26e-2913-2b0b-1352-63160096f78f@arm.com> <20180709112326.GD9486@e103592.cambridge.arm.com> From: Marc Zyngier Organization: ARM Ltd Message-ID: <17f8d585-3489-ab6f-6ee1-4d8d337dcf9c@arm.com> Date: Mon, 9 Jul 2018 13:29:42 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.8.0 MIME-Version: 1.0 In-Reply-To: <20180709112326.GD9486@e103592.cambridge.arm.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-GB Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 09/07/18 12:23, Dave Martin wrote: > On Fri, Jul 06, 2018 at 05:39:00PM +0100, Suzuki K Poulose wrote: >> On 07/06/2018 04:09 PM, Marc Zyngier wrote: >>> On 06/07/18 14:49, Suzuki K Poulose wrote: >>>> On 04/07/18 23:03, Suzuki K Poulose wrote: >>>>> On 07/04/2018 04:51 PM, Will Deacon wrote: >>>>>> Hi Suzuki, >>>>>> >>>>>> On Fri, Jun 29, 2018 at 12:15:35PM +0100, Suzuki K Poulose wrote: >>>>>>> Allow specifying the physical address size for a new VM via >>>>>>> the kvm_type argument for KVM_CREATE_VM ioctl. This allows >>>>>>> us to finalise the stage2 page table format as early as possible >>>>>>> and hence perform the right checks on the memory slots without >>>>>>> complication. The size is encoded as Log2(PA_Size) in the bits[7:0] >>>>>>> of the type field and can encode more information in the future if >>>>>>> required. The IPA size is still capped at 40bits. >>>>>>> >>>>>>> Cc: Marc Zyngier >>>>>>> Cc: Christoffer Dall >>>>>>> Cc: Peter Maydel >>>>>>> Cc: Paolo Bonzini >>>>>>> Cc: Radim Krčmář >>>>>>> Signed-off-by: Suzuki K Poulose >>>>>>> --- >>>>>>>   arch/arm/include/asm/kvm_mmu.h   |  2 ++ >>>>>>>   arch/arm64/include/asm/kvm_arm.h | 10 +++------- >>>>>>>   arch/arm64/include/asm/kvm_mmu.h |  2 ++ >>>>>>>   include/uapi/linux/kvm.h         | 10 ++++++++++ >>>>>>>   virt/kvm/arm/arm.c               | 24 ++++++++++++++++++++++-- >>>>>>>   5 files changed, 39 insertions(+), 9 deletions(-) >>>>>> >>>>>> [...] >>>>>> >>>>>>> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h >>>>>>> index 4df9bb6..fa4cab0 100644 >>>>>>> --- a/include/uapi/linux/kvm.h >>>>>>> +++ b/include/uapi/linux/kvm.h >>>>>>> @@ -751,6 +751,16 @@ struct kvm_ppc_resize_hpt { >>>>>>>   #define KVM_S390_SIE_PAGE_OFFSET 1 >>>>>>>   /* >>>>>>> + * On arm/arm64, machine type can be used to request the physical >>>>>>> + * address size for the VM. Bits [7-0] have been reserved for the >>>>>>> + * PA size shift (i.e, log2(PA_Size)). For backward compatibility, >>>>>>> + * value 0 implies the default IPA size, which is 40bits. >>>>>>> + */ >>>>>>> +#define KVM_VM_TYPE_ARM_PHYS_SHIFT_MASK    0xff >>>>>>> +#define KVM_VM_TYPE_ARM_PHYS_SHIFT(x)        \ >>>>>>> +    ((x) & KVM_VM_TYPE_ARM_PHYS_SHIFT_MASK) >>>>>> >>>>>> This seems like you're allocating quite a lot of bits in a non-extensible >>>>>> interface to a fairly esoteric parameter. Would it be better to add another >>>>>> ioctl, or condense the number of sizes you support instead? >>>>> >>>>> As I explained in the other thread, we need the size as soon as the VM >>>>> is created. The major challenge is keeping the backward compatibility by >>>>> mapping 0 to 40bits. I will give it a thought. >>>> >>>> Here is one option. We could re-use the {V}TCR_ELx.{I}PS field format, which >>>> occupies 3 bits and has the following definitions. (ID_AA64MMFR0_EL1:PARange >>>> also has the field definitions, except that the field is 4bits wide, but >>>> only 3bits are used) >>>> >>>> 000 32 bits, 4GB. >>>> 001 36 bits, 64GB. >>>> 010 40 bits, 1TB. >>>> 011 42 bits, 4TB. >>>> 100 44 bits, 16TB. >>>> 101 48 bits, 256TB. >>>> 110 52 bits, 4PB >>>> >>>> But we need to map 0 => 40bits IPA to make our ABI backward compatible. So >>>> we could use the additional one bit to indicate that IPA size is requested >>>> in the 3 bits. >>>> >>>> i.e, >>>> >>>> machine_type: >>>> >>>> Bit [2:0] - Requested IPA size. Values follow VTCR_EL2.PS format. >>>> >>>> Bit [3] - 1 => IPA Size bits (Bits[2:0]) requested. >>>> 0 => Not requested >>>> >>>> The only minor down side is restricting to the predefined values above, >>>> which is not a real issue for a VM. >>>> >>>> Thoughts ? >>> >>> I'd be very wary of using that 4th bit to do something that is not in >>> the architecture. We have only a single value left to be used (0b111), >>> and then your scheme clashes with the architecture definition. >> >> I agree. However, if we ever go beyond the 3bits in PARange, we have an >> issue with {V}TCR counter part. But lets not take that chance. >> >>> >>> I'd rather encode things in a way that is independent from the >>> architecture, and be done with it. You can map 0 to 40bits, and we have >>> the ability to express all values the architecture has (just in a >>> different order). >> >> The other option I can think of is encoding a signed number which is the >> difference of the IPA from 40. But that would need 5 bits if we were to >> encode it as it is. And if we want to squeeze it in 4bit, we could store >> half the difference (limiting the IPA limit to even numbers). >> >> i.e IPA = 40 + 2 * sign_extend(bits[3:0); > > I came across similar issues when trying to work out how to enable > SVE for KVM. In the end I reduced this to a per-vcpu feature, but > it means that there is no global opt-in for the SVE-specific KVM > API extensions: > > That's a bit gross, because SVE may require a change to the way > vcpus are initialised. The set of supported SVE vector lengths needs > to be set somehow before the vcpu is set running, but it's tricky do > do that without a new ioctl -- which would mean that if SVE is enabled > for a vcpu then the vcpu is not considered runnable until the new > magic ioctl is called. > > Opting into that semantic change globally at VM creation time might > be preferable. On the SVE side, this is still very much subject to > review/change. > > > Here: > > The KVM_CREATE_VM init argument seems undefined by the KVM core code and > is available for arches to abuse in creative ways. x86 and arm have > nothing here and reject non-zero values with -EINVAL; s390 treats it as > a bitmask, and defines a sincle feature-like bit here; powerpc treats it > as an enumeration of VM types. > > If we want to be extensible, we could > > a) Pass a pointer in type, and come up with some extensible VM parameter > struct for it to point to (which then wouldn't need a cryptic > compressed encoding), or > > b) Introduce a new "KVM_CREATE_VM2" variant that either takes such > an argument, or mandates a parameter negotiation phase involving > additional ioctls before marking the VM as ready for vcpu and > device creation. > > (a) feels like an easy backwards-compatible approach, but cannot be > readily adopted by other arches (maybe not an issue). > > (b) might be considered overengineered, so it would need a bit of > thought. > > Wedging arguments into a few bits in the type argument feels awkward, > and may be regretted later if we run out of bits, or something can't be > represented in the chosen encoding. I think that's a pretty convincing argument for a "better" CREATE_VM, one that would have a clearly defined, structured (and potentially extensible) argument. I've quickly hacked the following: diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h index b6270a3b38e9..3e76214034c2 100644 --- a/include/uapi/linux/kvm.h +++ b/include/uapi/linux/kvm.h @@ -735,6 +735,20 @@ struct kvm_ppc_resize_hpt { __u32 pad; }; +struct kvm_create_vm2 { + __u64 version; /* Or maybe not */ + union { + struct { +#define KVM_ARM_SVE_CAPABLE (1 << 0) +#define KVM_ARM_SELECT_IPA {1 << 1) + __u64 capabilities; + __u16 sve_vlen; + __u8 ipa_size; + } arm64; + __u64 dummy[15]; + }; +}; + #define KVMIO 0xAE /* machine type bits, to be used as argument to KVM_CREATE_VM */ Other architectures could fill in their own bits if they need to. Thoughts? M. -- Jazz is not dead. It just smells funny...