linux-kernel - Re: [PATCH v13 7/8] KVM: arm64: ioctl to fetch/store tags in a guest

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Date:   Fri, 4 Jun 2021 12:15:56 +0100
From:   Steven Price <steven.price@....com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Marc Zyngier <maz@...nel.org>, Will Deacon <will@...nel.org>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        kvmarm@...ts.cs.columbia.edu, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, Dave Martin <Dave.Martin@....com>,
        Mark Rutland <mark.rutland@....com>,
        Thomas Gleixner <tglx@...utronix.de>, qemu-devel@...gnu.org,
        Juan Quintela <quintela@...hat.com>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        Richard Henderson <richard.henderson@...aro.org>,
        Peter Maydell <peter.maydell@...aro.org>,
        Haibo Xu <Haibo.Xu@....com>, Andrew Jones <drjones@...hat.com>
Subject: Re: [PATCH v13 7/8] KVM: arm64: ioctl to fetch/store tags in a guest

On 03/06/2021 18:13, Catalin Marinas wrote:
> On Mon, May 24, 2021 at 11:45:12AM +0100, Steven Price wrote:
>> diff --git a/arch/arm64/include/uapi/asm/kvm.h b/arch/arm64/include/uapi/asm/kvm.h
>> index 24223adae150..b3edde68bc3e 100644
>> --- a/arch/arm64/include/uapi/asm/kvm.h
>> +++ b/arch/arm64/include/uapi/asm/kvm.h
>> @@ -184,6 +184,17 @@ struct kvm_vcpu_events {
>>  	__u32 reserved[12];
>>  };
>>  
>> +struct kvm_arm_copy_mte_tags {
>> +	__u64 guest_ipa;
>> +	__u64 length;
>> +	void __user *addr;
>> +	__u64 flags;
>> +	__u64 reserved[2];
>> +};
>> +
>> +#define KVM_ARM_TAGS_TO_GUEST		0
>> +#define KVM_ARM_TAGS_FROM_GUEST		1
>> +
>>  /* If you need to interpret the index values, here is the key: */
>>  #define KVM_REG_ARM_COPROC_MASK		0x000000000FFF0000
>>  #define KVM_REG_ARM_COPROC_SHIFT	16
>> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
>> index e89a5e275e25..baa33359e477 100644
>> --- a/arch/arm64/kvm/arm.c
>> +++ b/arch/arm64/kvm/arm.c
>> @@ -1345,6 +1345,13 @@ long kvm_arch_vm_ioctl(struct file *filp,
>>  
>>  		return 0;
>>  	}
>> +	case KVM_ARM_MTE_COPY_TAGS: {
>> +		struct kvm_arm_copy_mte_tags copy_tags;
>> +
>> +		if (copy_from_user(&copy_tags, argp, sizeof(copy_tags)))
>> +			return -EFAULT;
>> +		return kvm_vm_ioctl_mte_copy_tags(kvm, &copy_tags);
>> +	}
> 
> I wonder whether we need an update of the user structure following a
> fault, like how much was copied etc. In case of an error, some tags were
> copied and the VMM may want to skip the page before continuing. But here
> there's no such information provided.
> 
> On the ptrace interface, we return 0 on the syscall if any bytes were
> copied and update iov_len to such number. Maybe you want to still return
> an error here but updating copy_tags.length would be nice (and, of
> course, a copy_to_user() back).
> 

Good idea - as you suggest I'll make it update length with the number of
bytes not processed. Although in general I think we're expecting the VMM
to know where the memory is so this is more of a programming error - but
could still be useful for debugging.

Thanks,

Steve