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

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Message-ID: <20210604114233.GE31173@arm.com>
Date:   Fri, 4 Jun 2021 12:42:34 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Steven Price <steven.price@....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 Fri, Jun 04, 2021 at 12:15:56PM +0100, Steven Price wrote:
> 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.

Or update it to the number of bytes copied to be consistent with
ptrace()'s iov.len. On success, the structure is effectively left
unchanged.

-- 
Catalin