linux-kernel - Re: [PATCH v5 6/7] KVM: arm64: Mask FEAT

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Date:   Sat, 7 Jan 2023 18:53:28 +0900
From:   Akihiko Odaki <akihiko.odaki@...nix.com>
To:     Oliver Upton <oliver.upton@...ux.dev>
Cc:     Mark Brown <broonie@...nel.org>, Marc Zyngier <maz@...nel.org>,
        linux-kernel@...r.kernel.org, kvmarm@...ts.linux.dev,
        kvmarm@...ts.cs.columbia.edu, linux-arm-kernel@...ts.infradead.org,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        James Morse <james.morse@....com>,
        Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        asahi@...ts.linux.dev, Alyssa Rosenzweig <alyssa@...enzweig.io>,
        Sven Peter <sven@...npeter.dev>,
        Hector Martin <marcan@...can.st>
Subject: Re: [PATCH v5 6/7] KVM: arm64: Mask FEAT_CCIDX

On 2023/01/06 7:22, Oliver Upton wrote:
> On Fri, Dec 30, 2022 at 06:54:51PM +0900, Akihiko Odaki wrote:
>> The CCSIDR access handler masks the associativity bits according to the
>> bit layout for processors without FEAT_CCIDX. KVM also assumes CCSIDR is
>> 32-bit where it will be 64-bit if FEAT_CCIDX is enabled. Mask FEAT_CCIDX
>> so that these assumptions hold.
>>
>> Suggested-by: Marc Zyngier <maz@...nel.org>
>> Signed-off-by: Akihiko Odaki <akihiko.odaki@...nix.com>
> 
> FYI, I'm an idiot and replied to v4 of this patch... Forwarding comments
> below:
> 
>> ---
>>   arch/arm64/kvm/sys_regs.c | 11 +++++++++++
>>   1 file changed, 11 insertions(+)
>>
>> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
>> index f4a7c5abcbca..aeabf1f3370b 100644
>> --- a/arch/arm64/kvm/sys_regs.c
>> +++ b/arch/arm64/kvm/sys_regs.c
>> @@ -1124,6 +1124,12 @@ static u64 read_id_reg(const struct kvm_vcpu *vcpu, struct sys_reg_desc const *r
>>   						      ID_DFR0_PERFMON_SHIFT,
>>   						      kvm_vcpu_has_pmu(vcpu) ? ID_DFR0_PERFMON_8_4 : 0);
>>   		break;
>> +	case SYS_ID_AA64MMFR2_EL1:
>> +		val &= ~ID_AA64MMFR2_EL1_CCIDX_MASK;
>> +		break;
>> +	case SYS_ID_MMFR4_EL1:
>> +		val &= ~ARM64_FEATURE_MASK(ID_MMFR4_CCIDX);
>> +		break;
> 
> Not that it is necessarily worth addressing, but I wanted to point
> something out.
> 
> This change breaks migration from older kernels on implementations w/
> FEAT_CCIDX. There is most likely exactly 0 of those in the wild, but
> we need to be careful changing user-visible stuff like this.
> 
> --
> Thanks,
> Oliver

I also don't think whether FEAT_CCIDX is visible matters for any guest 
because the line size a guest would care is held in the same bits 
whether FEAT_CCIDX is implemented. But if it concerns you I can write a 
bit more code so that it won't mask CCIDX bit if it's set from the 
userspace.

Regards,
Akihiko Odaki