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Thu, 18 Apr 2019 01:48:21 -0700 (PDT) Received: from [10.1.196.92] (usa-sjc-imap-foss1.foss.arm.com [10.72.51.249]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 18B1E3F68F; Thu, 18 Apr 2019 01:48:16 -0700 (PDT) Subject: Re: [PATCH v9 1/5] KVM: arm64: Add a vcpu flag to control ptrauth for guest To: Dave Martin Cc: Catalin Marinas , Will Deacon , linux-kernel@vger.kernel.org, Kristina Martsenko , Ramana Radhakrishnan , Amit Daniel Kachhap , kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org References: <1555039236-10608-1-git-send-email-amit.kachhap@arm.com> <1555039236-10608-2-git-send-email-amit.kachhap@arm.com> <239c5d74-221e-cf8c-2c41-80db016bdc2b@arm.com> <20190417145255.GB3567@e103592.cambridge.arm.com> <20190417172010.GE3567@e103592.cambridge.arm.com> From: Marc Zyngier Openpgp: preference=signencrypt Autocrypt: addr=marc.zyngier@arm.com; prefer-encrypt=mutual; keydata= mQINBE6Jf0UBEADLCxpix34Ch3kQKA9SNlVQroj9aHAEzzl0+V8jrvT9a9GkK+FjBOIQz4KE g+3p+lqgJH4NfwPm9H5I5e3wa+Scz9wAqWLTT772Rqb6hf6kx0kKd0P2jGv79qXSmwru28vJ t9NNsmIhEYwS5eTfCbsZZDCnR31J6qxozsDHpCGLHlYym/VbC199Uq/pN5gH+5JHZyhyZiNW ozUCjMqC4eNW42nYVKZQfbj/k4W9xFfudFaFEhAf/Vb1r6F05eBP1uopuzNkAN7vqS8XcgQH qXI357YC4ToCbmqLue4HK9+2mtf7MTdHZYGZ939OfTlOGuxFW+bhtPQzsHiW7eNe0ew0+LaL 3wdNzT5abPBscqXWVGsZWCAzBmrZato+Pd2bSCDPLInZV0j+rjt7MWiSxEAEowue3IcZA++7 ifTDIscQdpeKT8hcL+9eHLgoSDH62SlubO/y8bB1hV8JjLW/jQpLnae0oz25h39ij4ijcp8N t5slf5DNRi1NLz5+iaaLg4gaM3ywVK2VEKdBTg+JTg3dfrb3DH7ctTQquyKun9IVY8AsxMc6 lxl4HxrpLX7HgF10685GG5fFla7R1RUnW5svgQhz6YVU33yJjk5lIIrrxKI/wLlhn066mtu1 DoD9TEAjwOmpa6ofV6rHeBPehUwMZEsLqlKfLsl0PpsJwov8TQARAQABtCNNYXJjIFp5bmdp ZXIgPG1hcmMuenluZ2llckBhcm0uY29tPokCOwQTAQIAJQIbAwYLCQgHAwIGFQgCCQoLBBYC AwECHgECF4AFAk6NvYYCGQEACgkQI9DQutE9ekObww/+NcUATWXOcnoPflpYG43GZ0XjQLng LQFjBZL+CJV5+1XMDfz4ATH37cR+8gMO1UwmWPv5tOMKLHhw6uLxGG4upPAm0qxjRA/SE3LC 22kBjWiSMrkQgv5FDcwdhAcj8A+gKgcXBeyXsGBXLjo5UQOGvPTQXcqNXB9A3ZZN9vS6QUYN TXFjnUnzCJd+PVI/4jORz9EUVw1q/+kZgmA8/GhfPH3xNetTGLyJCJcQ86acom2liLZZX4+1 6Hda2x3hxpoQo7pTu+XA2YC4XyUstNDYIsE4F4NVHGi88a3N8yWE+Z7cBI2HjGvpfNxZnmKX 6bws6RQ4LHDPhy0yzWFowJXGTqM/e79c1UeqOVxKGFF3VhJJu1nMlh+5hnW4glXOoy/WmDEM UMbl9KbJUfo+GgIQGMp8mwgW0vK4HrSmevlDeMcrLdfbbFbcZLNeFFBn6KqxFZaTd+LpylIH bOPN6fy1Dxf7UZscogYw5Pt0JscgpciuO3DAZo3eXz6ffj2NrWchnbj+SpPBiH4srfFmHY+Y LBemIIOmSqIsjoSRjNEZeEObkshDVG5NncJzbAQY+V3Q3yo9og/8ZiaulVWDbcpKyUpzt7pv cdnY3baDE8ate/cymFP5jGJK++QCeA6u6JzBp7HnKbngqWa6g8qDSjPXBPCLmmRWbc5j0lvA 6ilrF8m5Ag0ETol/RQEQAM/2pdLYCWmf3rtIiP8Wj5NwyjSL6/UrChXtoX9wlY8a4h3EX6E3 64snIJVMLbyr4bwdmPKULlny7T/R8dx/mCOWu/DztrVNQiXWOTKJnd/2iQblBT+W5W8ep/nS w3qUIckKwKdplQtzSKeE+PJ+GMS+DoNDDkcrVjUnsoCEr0aK3cO6g5hLGu8IBbC1CJYSpple VVb/sADnWF3SfUvJ/l4K8Uk4B4+X90KpA7U9MhvDTCy5mJGaTsFqDLpnqp/yqaT2P7kyMG2E w+eqtVIqwwweZA0S+tuqput5xdNAcsj2PugVx9tlw/LJo39nh8NrMxAhv5aQ+JJ2I8UTiHLX QvoC0Yc/jZX/JRB5r4x4IhK34Mv5TiH/gFfZbwxd287Y1jOaD9lhnke1SX5MXF7eCT3cgyB+ hgSu42w+2xYl3+rzIhQqxXhaP232t/b3ilJO00ZZ19d4KICGcakeiL6ZBtD8TrtkRiewI3v0 o8rUBWtjcDRgg3tWx/PcJvZnw1twbmRdaNvsvnlapD2Y9Js3woRLIjSAGOijwzFXSJyC2HU1 AAuR9uo4/QkeIrQVHIxP7TJZdJ9sGEWdeGPzzPlKLHwIX2HzfbdtPejPSXm5LJ026qdtJHgz BAb3NygZG6BH6EC1NPDQ6O53EXorXS1tsSAgp5ZDSFEBklpRVT3E0NrDABEBAAGJAh8EGAEC AAkFAk6Jf0UCGwwACgkQI9DQutE9ekMLBQ//U+Mt9DtFpzMCIHFPE9nNlsCm75j22lNiw6mX mx3cUA3pl+uRGQr/zQC5inQNtjFUmwGkHqrAw+SmG5gsgnM4pSdYvraWaCWOZCQCx1lpaCOl MotrNcwMJTJLQGc4BjJyOeSH59HQDitKfKMu/yjRhzT8CXhys6R0kYMrEN0tbe1cFOJkxSbV 0GgRTDF4PKyLT+RncoKxQe8lGxuk5614aRpBQa0LPafkirwqkUtxsPnarkPUEfkBlnIhAR8L kmneYLu0AvbWjfJCUH7qfpyS/FRrQCoBq9QIEcf2v1f0AIpA27f9KCEv5MZSHXGCdNcbjKw1 39YxYZhmXaHFKDSZIC29YhQJeXWlfDEDq6nIhvurZy3mSh2OMQgaIoFexPCsBBOclH8QUtMk a3jW/qYyrV+qUq9Wf3SKPrXf7B3xB332jFCETbyZQXqmowV+2b3rJFRWn5hK5B+xwvuxKyGq qDOGjof2dKl2zBIxbFgOclV7wqCVkhxSJi/QaOj2zBqSNPXga5DWtX3ekRnJLa1+ijXxmdjz hApihi08gwvP5G9fNGKQyRETePEtEAWt0b7dOqMzYBYGRVr7uS4uT6WP7fzOwAJC4lU7ZYWZ yVshCa0IvTtp1085RtT3qhh9mobkcZ+7cQOY+Tx2RGXS9WeOh2jZjdoWUv6CevXNQyOUXMM= Organization: ARM Ltd Message-ID: Date: Thu, 18 Apr 2019 09:48:14 +0100 User-Agent: Mozilla/5.0 (X11; Linux aarch64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 MIME-Version: 1.0 In-Reply-To: <20190417172010.GE3567@e103592.cambridge.arm.com> Content-Type: text/plain; charset=utf-8 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 On 17/04/2019 18:20, Dave Martin wrote: > On Wed, Apr 17, 2019 at 04:54:32PM +0100, Marc Zyngier wrote: >> On 17/04/2019 15:52, Dave Martin wrote: >>> On Wed, Apr 17, 2019 at 03:19:11PM +0100, Marc Zyngier wrote: >>>> On 17/04/2019 14:08, Amit Daniel Kachhap wrote: >>>>> Hi, >>>>> >>>>> On 4/17/19 2:05 PM, Marc Zyngier wrote: >>>>>> On 12/04/2019 04:20, Amit Daniel Kachhap wrote: >>>>>>> A per vcpu flag is added to check if pointer authentication is >>>>>>> enabled for the vcpu or not. This flag may be enabled according to >>>>>>> the necessary user policies and host capabilities. >>>>>>> >>>>>>> This patch also adds a helper to check the flag. >>>>>>> >>>>>>> Signed-off-by: Amit Daniel Kachhap >>>>>>> Cc: Mark Rutland >>>>>>> Cc: Marc Zyngier >>>>>>> Cc: Christoffer Dall >>>>>>> Cc: kvmarm@lists.cs.columbia.edu >>>>>>> --- >>>>>>> >>>>>>> Changes since v8: >>>>>>> * Added a new per vcpu flag which will store Pointer Authentication enable >>>>>>> status instead of checking them again. [Dave Martin] >>>>>>> >>>>>>> arch/arm64/include/asm/kvm_host.h | 4 ++++ >>>>>>> 1 file changed, 4 insertions(+) >>>>>>> >>>>>>> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h >>>>>>> index 9d57cf8..31dbc7c 100644 >>>>>>> --- a/arch/arm64/include/asm/kvm_host.h >>>>>>> +++ b/arch/arm64/include/asm/kvm_host.h >>>>>>> @@ -355,10 +355,14 @@ struct kvm_vcpu_arch { >>>>>>> #define KVM_ARM64_HOST_SVE_ENABLED (1 << 4) /* SVE enabled for EL0 */ >>>>>>> #define KVM_ARM64_GUEST_HAS_SVE (1 << 5) /* SVE exposed to guest */ >>>>>>> #define KVM_ARM64_VCPU_SVE_FINALIZED (1 << 6) /* SVE config completed */ >>>>>>> +#define KVM_ARM64_GUEST_HAS_PTRAUTH (1 << 7) /* PTRAUTH exposed to guest */ >>>>>>> >>>>>>> #define vcpu_has_sve(vcpu) (system_supports_sve() && \ >>>>>>> ((vcpu)->arch.flags & KVM_ARM64_GUEST_HAS_SVE)) >>>>>>> >>>>>>> +#define vcpu_has_ptrauth(vcpu) \ >>>>>>> + ((vcpu)->arch.flags & KVM_ARM64_GUEST_HAS_PTRAUTH) >>>>>>> + >>>>>> >>>>>> Just as for SVE, please first check that the system has PTRAUTH. >>>>>> Something like: >>>>>> >>>>>> (cpus_have_const_cap(ARM64_HAS_GENERIC_AUTH_ARCH) && \ >>>>>> ((vcpu)->arch.flags & KVM_ARM64_GUEST_HAS_PTRAUTH)) >>>>> >>>>> In the subsequent patches, vcpu->arch.flags is only set to >>>>> KVM_ARM64_GUEST_HAS_PTRAUTH when all host capability check conditions >>>>> matches such as system_supports_address_auth(), >>>>> system_supports_generic_auth() so doing them again is repetitive in my view. >>>> >>>> It isn't the setting of the flag I care about, but the check of that >>>> flag. Checking a flag for a feature that cannot be used on the running >>>> system should have a zero cost, which isn't the case here. >>>> >>>> Granted, the impact should be minimal and it looks like it mostly happen >>>> on the slow path, but at the very least it would be consistent. So even >>>> if you don't buy my argument about efficiency, please change it in the >>>> name of consistency. >>> >>> One of the annoyances here is there is no single static key for ptrauth. >>> >>> I'm assuming we don't want to check both static keys (for address and >>> generic auth) on hot paths. >> >> They both just branches, so I don't see why not. Of course, for people >> using a lesser compiler (gcc 4.8 or clang), things will suck. But they >> got it coming anyway. > > I seem to recall Christoffer expressing concerns about this at some > point: even unconditional branches branches to a fixed address are not > free (or even correctly predicted). Certainly not free, but likely less expensive than a load followed by a conditional branch. And actually, this is not a comparison against a branch, but against a nop. > I don't think any compiler can elide static key checks of merge them > together. It is not about eliding them, it is about having a cheap fast path. Compiling this: bool kvm_hack_test_static_key(struct kvm_vcpu *vcpu) { return ((system_supports_address_auth() || system_supports_generic_auth()) && vcpu->arch.flags & (1 << 6)); } I get: [...] ffff0000100db5c8: 1400000c b ffff0000100db5f8 ffff0000100db5cc: d503201f nop ffff0000100db5d0: 14000012 b ffff0000100db618 ffff0000100db5d4: d503201f nop ffff0000100db5d8: 14000014 b ffff0000100db628 ffff0000100db5dc: d503201f nop ffff0000100db5e0: 14000017 b ffff0000100db63c ffff0000100db5e4: d503201f nop ffff0000100db5e8: 52800000 mov w0, #0x0 // #0 ffff0000100db5ec: f9400bf3 ldr x19, [sp, #16] ffff0000100db5f0: a8c27bfd ldp x29, x30, [sp], #32 ffff0000100db5f4: d65f03c0 ret ffff0000100db5f8: b000ac40 adrp x0, ffff000011664000 ffff0000100db5fc: f942a400 ldr x0, [x0, #1352] ffff0000100db600: b637fe80 tbz x0, #38, ffff0000100db5d0 ffff0000100db604: f9441660 ldr x0, [x19, #2088] ffff0000100db608: f9400bf3 ldr x19, [sp, #16] ffff0000100db60c: 53061800 ubfx w0, w0, #6, #1 ffff0000100db610: a8c27bfd ldp x29, x30, [sp], #32 ffff0000100db614: d65f03c0 ret ffff0000100db618: b000ac40 adrp x0, ffff000011664000 ffff0000100db61c: f942a400 ldr x0, [x0, #1352] ffff0000100db620: b73fff20 tbnz x0, #39, ffff0000100db604 ffff0000100db624: 17ffffed b ffff0000100db5d8 ffff0000100db628: b000ac40 adrp x0, ffff000011664000 ffff0000100db62c: f942a400 ldr x0, [x0, #1352] ffff0000100db630: b747fea0 tbnz x0, #40, ffff0000100db604 ffff0000100db634: 14000002 b ffff0000100db63c ffff0000100db638: 17ffffeb b ffff0000100db5e4 ffff0000100db63c: b000ac40 adrp x0, ffff000011664000 ffff0000100db640: f942a400 ldr x0, [x0, #1352] ffff0000100db644: b74ffe00 tbnz x0, #41, ffff0000100db604 ffff0000100db648: 52800000 mov w0, #0x0 // #0 ffff0000100db64c: 17ffffe8 b ffff0000100db5ec Once the initial 4 branches that are there to deal with the pre static keys checks are nop-ed, everything is controlled by the remaining 4 nops which are turned into branches to ffff0000100db604 if any of the conditions become true. Which is exactly what we want: a fall through to returning zero without doing anything else. Thanks, M. > Maybe I am misremembering. > > Cheers > ---Dave > -- Jazz is not dead. It just smells funny...