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;
Subject: Re: [PATCH v9 1/5] KVM: arm64: Add a vcpu flag to control ptrauth for
 guest
To:     Dave Martin <Dave.Martin@arm.com>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        linux-kernel@vger.kernel.org,
        Kristina Martsenko <kristina.martsenko@arm.com>,
        Ramana Radhakrishnan <ramana.radhakrishnan@arm.com>,
        Amit Daniel Kachhap <amit.kachhap@arm.com>,
        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>
 <eeeebc4d-f061-42a5-954d-40161bbed23a@arm.com>
 <ef1eb326-cd3a-4033-813d-ceae262b4e23@arm.com>
 <239c5d74-221e-cf8c-2c41-80db016bdc2b@arm.com>
 <20190417145255.GB3567@e103592.cambridge.arm.com>
 <a04c6bfd-fd6a-8ff9-a66a-3391af53afb5@arm.com>
 <20190417172010.GE3567@e103592.cambridge.arm.com>
From:   Marc Zyngier <marc.zyngier@arm.com>
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: <bec616ee-2765-4243-2108-84f3230c6aa0@arm.com>
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

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 <amit.kachhap@arm.com>
>>>>>>> Cc: Mark Rutland <mark.rutland@arm.com>
>>>>>>> Cc: Marc Zyngier <marc.zyngier@arm.com>
>>>>>>> Cc: Christoffer Dall <christoffer.dall@arm.com>
>>>>>>> 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 <kvm_hack_test_static_key+0x48>
ffff0000100db5cc:       d503201f        nop
ffff0000100db5d0:       14000012        b       ffff0000100db618 <kvm_hack_test_static_key+0x68>
ffff0000100db5d4:       d503201f        nop
ffff0000100db5d8:       14000014        b       ffff0000100db628 <kvm_hack_test_static_key+0x78>
ffff0000100db5dc:       d503201f        nop
ffff0000100db5e0:       14000017        b       ffff0000100db63c <kvm_hack_test_static_key+0x8c>
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 <reset_devices>
ffff0000100db5fc:       f942a400        ldr     x0, [x0, #1352]
ffff0000100db600:       b637fe80        tbz     x0, #38, ffff0000100db5d0 <kvm_hack_test_static_key+0x20>
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 <reset_devices>
ffff0000100db61c:       f942a400        ldr     x0, [x0, #1352]
ffff0000100db620:       b73fff20        tbnz    x0, #39, ffff0000100db604 <kvm_hack_test_static_key+0x54>
ffff0000100db624:       17ffffed        b       ffff0000100db5d8 <kvm_hack_test_static_key+0x28>
ffff0000100db628:       b000ac40        adrp    x0, ffff000011664000 <reset_devices>
ffff0000100db62c:       f942a400        ldr     x0, [x0, #1352]
ffff0000100db630:       b747fea0        tbnz    x0, #40, ffff0000100db604 <kvm_hack_test_static_key+0x54>
ffff0000100db634:       14000002        b       ffff0000100db63c <kvm_hack_test_static_key+0x8c>
ffff0000100db638:       17ffffeb        b       ffff0000100db5e4 <kvm_hack_test_static_key+0x34>
ffff0000100db63c:       b000ac40        adrp    x0, ffff000011664000 <reset_devices>
ffff0000100db640:       f942a400        ldr     x0, [x0, #1352]
ffff0000100db644:       b74ffe00        tbnz    x0, #41, ffff0000100db604 <kvm_hack_test_static_key+0x54>
ffff0000100db648:       52800000        mov     w0, #0x0                        // #0
ffff0000100db64c:       17ffffe8        b       ffff0000100db5ec <kvm_hack_test_static_key+0x3c>

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...