linux-kernel - Re: [PATCH 2/2] arm64: Support systems without FP/ASIMD

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Message-ID: <a29f3668-4caf-5c88-82c0-8e2208fb285c@arm.com>
Date:   Tue, 25 Oct 2016 17:48:36 +0100
From:   Suzuki K Poulose <Suzuki.Poulose@....com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
CC:     "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Marc Zyngier <marc.zyngier@....com>,
        Christoffer Dall <christoffer.dall@...aro.org>,
        Mark Rutland <mark.rutland@....com>,
        "kvmarm@...ts.cs.columbia.edu" <kvmarm@...ts.cs.columbia.edu>,
        KVM devel mailing list <kvm@...r.kernel.org>
Subject: Re: [PATCH 2/2] arm64: Support systems without FP/ASIMD

On 25/10/16 15:00, Ard Biesheuvel wrote:
> Hi Suzuki,
>
> On 25 October 2016 at 14:50, Suzuki K Poulose <suzuki.poulose@....com> wrote:
>> The arm64 kernel assumes that FP/ASIMD units are always present
>> and accesses the FP/ASIMD specific registers unconditionally. This
>> could cause problems when they are absent. This patch adds the
>> support for kernel handling systems without FP/ASIMD by skipping the
>> register access within the kernel. For kvm, we trap the accesses
>> to FP/ASIMD and inject an undefined instruction exception to the VM.
>>
>> The callers of the exported kernel_neon_begin_parital() should
>> make sure that the FP/ASIMD is supported.
>>
>> Cc: Catalin Marinas <catalin.marinas@....com>
>> Cc: Will Deacon <will.deacon@....com>
>> Cc: Christoffer Dall <christoffer.dall@...aro.org>
>> Cc: Marc Zyngier <marc.zyngier@....com>
>> Cc: Ard Biesheuvel <ard.biesheuvel@...aro.org>
>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
>> ---
>> ---
>>  arch/arm64/crypto/aes-ce-ccm-glue.c |  2 +-
>>  arch/arm64/crypto/aes-ce-cipher.c   |  2 ++
>>  arch/arm64/crypto/ghash-ce-glue.c   |  2 ++
>>  arch/arm64/crypto/sha1-ce-glue.c    |  2 ++
>>  arch/arm64/include/asm/cpufeature.h |  8 +++++++-
>>  arch/arm64/include/asm/neon.h       |  3 ++-
>>  arch/arm64/kernel/cpufeature.c      | 15 +++++++++++++++
>>  arch/arm64/kernel/fpsimd.c          | 14 ++++++++++++++
>>  arch/arm64/kvm/handle_exit.c        | 11 +++++++++++
>>  arch/arm64/kvm/hyp/hyp-entry.S      |  9 ++++++++-
>>  arch/arm64/kvm/hyp/switch.c         |  5 ++++-
>>  11 files changed, 68 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/arm64/crypto/aes-ce-ccm-glue.c b/arch/arm64/crypto/aes-ce-ccm-glue.c
>> index f4bf2f2..d001b4e 100644
>> --- a/arch/arm64/crypto/aes-ce-ccm-glue.c
>> +++ b/arch/arm64/crypto/aes-ce-ccm-glue.c
>> @@ -296,7 +296,7 @@ static struct aead_alg ccm_aes_alg = {
>>
>>  static int __init aes_mod_init(void)
>>  {
>> -       if (!(elf_hwcap & HWCAP_AES))
>> +       if (!(elf_hwcap & HWCAP_AES) || !system_supports_fpsimd())
>
> This looks weird to me. All crypto extensionsinstructions except CRC
> operate strictly on FP/ASIMD registers, and so support for FP/ASIMD is
> implied by having HWCAP_AES. In other words, I think it makes more
> sense to sanity check that the info registers are consistent with each
> other in core code than modifying each user (which for HWCAP_xxx
> includes userland) to double check that the world is sane.

You're right. I will respin it.
Btw, I think we need the following feature check for the above. I will send
that in and remove the explicit HWCAP_AES check.

module_cpu_feature_match(AES, aes_mod_init());

Cheers
Suzuki
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