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Message-ID: <9f90acc8-54b1-460a-dc25-d2d84e70c9d1@arm.com>
Date: Wed, 10 Mar 2021 16:47:56 +0000
From: Steven Price <steven.price@....com>
To: Marc Zyngier <maz@...nel.org>
Cc: Catalin Marinas <catalin.marinas@....com>,
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 v9 5/6] KVM: arm64: ioctl to fetch/store tags in a guest
On 09/03/2021 17:57, Marc Zyngier wrote:
> On Mon, 01 Mar 2021 14:23:14 +0000,
> Steven Price <steven.price@....com> wrote:
>>
>> The VMM may not wish to have it's own mapping of guest memory mapped
>> with PROT_MTE because this causes problems if the VMM has tag checking
>> enabled (the guest controls the tags in physical RAM and it's unlikely
>> the tags are correct for the VMM).
>>
>> Instead add a new ioctl which allows the VMM to easily read/write the
>> tags from guest memory, allowing the VMM's mapping to be non-PROT_MTE
>> while the VMM can still read/write the tags for the purpose of
>> migration.
>>
>> Signed-off-by: Steven Price <steven.price@....com>
>> ---
>> arch/arm64/include/uapi/asm/kvm.h | 13 +++++++
>> arch/arm64/kvm/arm.c | 57 +++++++++++++++++++++++++++++++
>> include/uapi/linux/kvm.h | 1 +
>> 3 files changed, 71 insertions(+)
>>
>> diff --git a/arch/arm64/include/uapi/asm/kvm.h b/arch/arm64/include/uapi/asm/kvm.h
>> index 24223adae150..5fc2534ac5df 100644
>> --- a/arch/arm64/include/uapi/asm/kvm.h
>> +++ b/arch/arm64/include/uapi/asm/kvm.h
>> @@ -184,6 +184,19 @@ struct kvm_vcpu_events {
>> __u32 reserved[12];
>> };
>>
>> +struct kvm_arm_copy_mte_tags {
>> + __u64 guest_ipa;
>> + __u64 length;
>> + union {
>> + void __user *addr;
>> + __u64 padding;
>> + };
>> + __u64 flags;
>
> I'd be keen on a couple of reserved __64s. Just in case...
Fair enough, I'll add a __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 46bf319f6cb7..01d404833e24 100644
>> --- a/arch/arm64/kvm/arm.c
>> +++ b/arch/arm64/kvm/arm.c
>> @@ -1297,6 +1297,53 @@ static int kvm_vm_ioctl_set_device_addr(struct kvm *kvm,
>> }
>> }
>>
>> +static int kvm_vm_ioctl_mte_copy_tags(struct kvm *kvm,
>> + struct kvm_arm_copy_mte_tags *copy_tags)
>> +{
>> + gpa_t guest_ipa = copy_tags->guest_ipa;
>> + size_t length = copy_tags->length;
>> + void __user *tags = copy_tags->addr;
>> + gpa_t gfn;
>> + bool write = !(copy_tags->flags & KVM_ARM_TAGS_FROM_GUEST);
>> +
>> + if (copy_tags->flags & ~KVM_ARM_TAGS_FROM_GUEST)
>> + return -EINVAL;
>> +
>> + if (length & ~PAGE_MASK || guest_ipa & ~PAGE_MASK)
>> + return -EINVAL;
>
> It is a bit odd to require userspace to provide a page-aligned
> addr/size, as it now has to find out about the kernel's page
> size. MTE_GRANULE_SIZE-aligned values would make more sense. Is there
> an underlying reason for this?
No fundamental reason, my thoughts were:
* It's likely user space is naturally going to be using page-aligned
quantities during migration, so it already has to care about this.
* It makes the loop below easier.
* It's easy to relax the restriction in the future if it becomes a
problem, much harder to tighten it without breaking anything.
But I can switch to MTE_GRANULE_SIZE if you'd prefer, let me know.
>> +
>> + gfn = gpa_to_gfn(guest_ipa);
>> +
>> + while (length > 0) {
>> + kvm_pfn_t pfn = gfn_to_pfn_prot(kvm, gfn, write, NULL);
>> + void *maddr;
>> + unsigned long num_tags = PAGE_SIZE / MTE_GRANULE_SIZE;
>> +
>> + if (is_error_noslot_pfn(pfn))
>> + return -ENOENT;
>> +
>> + maddr = page_address(pfn_to_page(pfn));
>> +
>> + if (!write) {
>> + num_tags = mte_copy_tags_to_user(tags, maddr, num_tags);
>> + kvm_release_pfn_clean(pfn);
>> + } else {
>> + num_tags = mte_copy_tags_from_user(maddr, tags,
>> + num_tags);
>> + kvm_release_pfn_dirty(pfn);
>> + }
>> +
>
> Is it actually safe to do this without holding any lock, without
> checking anything against the mmu_notifier_seq? What if the pages are
> being swapped out? Or the memslot removed from under your feet?
>
> It looks... dangerous. Do you even want to allow this while vcpus are
> actually running?
Umm... yeah I'm not sure how I managed to forgot the locks. This should
be holding kvm->slots_lock to prevent the slot going under our feet. I
was surprised that lockdep didn't catch that, until I noticed I'd
disabled it and discovered why (the model makes it incredibly slow).
However I've done a run with it enabled now - and with the
kvm->slots_lock taken it's happy.
gfn_to_pfn_prot() internally calls a variant of get_user_pages() - so
swapping out shouldn't be a problem.
In terms of running with the vcpus running - given this is going to be
used for migration I think that's pretty much a requirement. We want to
be able to dump the tags while executing to enable early transfer of the
memory.
Steve
>> + if (num_tags != PAGE_SIZE / MTE_GRANULE_SIZE)
>> + return -EFAULT;
>> +
>> + gfn++;
>> + tags += num_tags;
>> + length -= PAGE_SIZE;
>> + }
>> +
>> + return 0;
>> +}
>> +
>> long kvm_arch_vm_ioctl(struct file *filp,
>> unsigned int ioctl, unsigned long arg)
>> {
>> @@ -1333,6 +1380,16 @@ 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 (!kvm_has_mte(kvm))
>> + return -EINVAL;
>> +
>> + if (copy_from_user(©_tags, argp, sizeof(copy_tags)))
>> + return -EFAULT;
>> + return kvm_vm_ioctl_mte_copy_tags(kvm, ©_tags);
>> + }
>> default:
>> return -EINVAL;
>> }
>> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
>> index 05618a4abf7e..b75af0f9ba55 100644
>> --- a/include/uapi/linux/kvm.h
>> +++ b/include/uapi/linux/kvm.h
>> @@ -1423,6 +1423,7 @@ struct kvm_s390_ucas_mapping {
>> /* Available with KVM_CAP_PMU_EVENT_FILTER */
>> #define KVM_SET_PMU_EVENT_FILTER _IOW(KVMIO, 0xb2, struct kvm_pmu_event_filter)
>> #define KVM_PPC_SVM_OFF _IO(KVMIO, 0xb3)
>> +#define KVM_ARM_MTE_COPY_TAGS _IOR(KVMIO, 0xb4, struct kvm_arm_copy_mte_tags)
>>
>> /* ioctl for vm fd */
>> #define KVM_CREATE_DEVICE _IOWR(KVMIO, 0xe0, struct kvm_create_device)
>> --
>> 2.20.1
>>
>>
>
> Thanks,
>
> M.
>
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