linux-kernel - Re: [PATCH 2/4] KVM: arm64: maintain per VM value for CTR

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Message-ID: <7e9c8367-6e9e-e36b-934e-170c90522134@redhat.com>
Date: Sat, 13 Apr 2024 15:05:30 +0200 (CEST)
From: Sebastian Ott <sebott@...hat.com>
To: Marc Zyngier <maz@...nel.org>
cc: linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev, 
    linux-kernel@...r.kernel.org, Oliver Upton <oliver.upton@...ux.dev>, 
    James Morse <james.morse@....com>, 
    Suzuki K Poulose <suzuki.poulose@....com>, 
    Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>
Subject: Re: [PATCH 2/4] KVM: arm64: maintain per VM value for CTR_EL0

On Sat, 13 Apr 2024, Marc Zyngier wrote:
> On Fri, 05 Apr 2024 13:01:06 +0100,
> Sebastian Ott <sebott@...hat.com> wrote:
>>
>> In preparation for CTR_EL0 emulation maintain a per VM for this
>> register and use it where appropriate.
>>
>> Signed-off-by: Sebastian Ott <sebott@...hat.com>
>> ---
>>  arch/arm64/include/asm/kvm_host.h |  1 +
>>  arch/arm64/kvm/sys_regs.c         | 22 +++++++++++++++-------
>>  2 files changed, 16 insertions(+), 7 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
>> index 9e8a496fb284..481216febb46 100644
>> --- a/arch/arm64/include/asm/kvm_host.h
>> +++ b/arch/arm64/include/asm/kvm_host.h
>> @@ -318,6 +318,7 @@ struct kvm_arch {
>>
>>  	/* PMCR_EL0.N value for the guest */
>>  	u8 pmcr_n;
>> +	u64 ctr_el0;
>>
>>  	/* Iterator for idreg debugfs */
>>  	u8	idreg_debugfs_iter;
>
> Please consider the alignment of the fields. This leaves a 7 byte hole
> that could be avoided (yes, I'm on a mission to reduce the size of the
> various structures, because they are absolute pigs).

OK.

>> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
>> index 131f5b0ca2b9..4d29b1a0842d 100644
>> --- a/arch/arm64/kvm/sys_regs.c
>> +++ b/arch/arm64/kvm/sys_regs.c
>> @@ -215,13 +215,21 @@ void vcpu_write_sys_reg(struct kvm_vcpu *vcpu, u64 val, int reg)
>>  /* CSSELR values; used to index KVM_REG_ARM_DEMUX_ID_CCSIDR */
>>  #define CSSELR_MAX 14
>>
>> +static u64 kvm_get_ctr_el0(struct kvm *kvm)
>> +{
>> +	if (kvm->arch.ctr_el0)
>> +		return kvm->arch.ctr_el0;
>
> Is this relying on some bits not being 0?
>
>> +
>> +	return read_sanitised_ftr_reg(SYS_CTR_EL0);
>
> Why isn't the shadow value always populated?

The idea was for kvm->arch.ctr_el0 being non zero only if userspace
set it up to differ from the host value. So it can be used to decide
if we need to set up a trap for the reg access (without comparing it
to the host value again).

Thanks,
Sebastian