Re: [RFC PATCH 3/6] KVM: SVM: Implement demand page pinning

[David Hildenbrand]

On 28.01.22 07:57, Nikunj A. Dadhania wrote:
> On 1/26/2022 4:16 PM, David Hildenbrand wrote:
>> On 18.01.22 12:06, Nikunj A Dadhania wrote:
>>> Use the memslot metadata to store the pinned data along with the pfns.
>>> This improves the SEV guest startup time from O(n) to a constant by
>>> deferring guest page pinning until the pages are used to satisfy nested
>>> page faults. The page reference will be dropped in the memslot free
>>> path.
>>>
>>> Remove the enc_region structure definition and the code which did
>>> upfront pinning, as they are no longer needed in view of the demand
>>> pinning support.
>>>
>>> Leave svm_register_enc_region() and svm_unregister_enc_region() as stubs
>>> since qemu is dependent on this API.
>>>
>>> Signed-off-by: Nikunj A Dadhania <nikunj@xxxxxxx>
>>> ---
>>>  arch/x86/kvm/svm/sev.c | 208 ++++++++++++++++-------------------------
>>>  arch/x86/kvm/svm/svm.c |   1 +
>>>  arch/x86/kvm/svm/svm.h |   3 +-
>>>  3 files changed, 81 insertions(+), 131 deletions(-)
>>>
>>> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
>>> index d972ab4956d4..a962bed97a0b 100644
>>> --- a/arch/x86/kvm/svm/sev.c
>>> +++ b/arch/x86/kvm/svm/sev.c
>>> @@ -66,14 +66,6 @@ static unsigned int nr_asids;
>>>  static unsigned long *sev_asid_bitmap;
>>>  static unsigned long *sev_reclaim_asid_bitmap;
>>>  
>>> -struct enc_region {
>>> -	struct list_head list;
>>> -	unsigned long npages;
>>> -	struct page **pages;
>>> -	unsigned long uaddr;
>>> -	unsigned long size;
>>> -};
>>> -
>>>  /* Called with the sev_bitmap_lock held, or on shutdown  */
>>>  static int sev_flush_asids(int min_asid, int max_asid)
>>>  {
>>> @@ -257,8 +249,6 @@ static int sev_guest_init(struct kvm *kvm, struct kvm_sev_cmd *argp)
>>>  	if (ret)
>>>  		goto e_free;
>>>  
>>> -	INIT_LIST_HEAD(&sev->regions_list);
>>> -
>>>  	return 0;
>>>  
>>>  e_free:
>>> @@ -1637,8 +1627,6 @@ static void sev_migrate_from(struct kvm_sev_info *dst,
>>>  	src->handle = 0;
>>>  	src->pages_locked = 0;
>>>  	src->enc_context_owner = NULL;
>>> -
>>> -	list_cut_before(&dst->regions_list, &src->regions_list, &src->regions_list);
>>>  }
>>>  
>>>  static int sev_es_migrate_from(struct kvm *dst, struct kvm *src)
>>> @@ -1861,115 +1849,13 @@ int svm_mem_enc_op(struct kvm *kvm, void __user *argp)
>>>  int svm_register_enc_region(struct kvm *kvm,
>>>  			    struct kvm_enc_region *range)
>>>  {
>>> -	struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
>>> -	struct enc_region *region;
>>> -	int ret = 0;
>>> -
>>> -	if (!sev_guest(kvm))
>>> -		return -ENOTTY;
>>> -
>>> -	/* If kvm is mirroring encryption context it isn't responsible for it */
>>> -	if (is_mirroring_enc_context(kvm))
>>> -		return -EINVAL;
>>> -
>>> -	if (range->addr > ULONG_MAX || range->size > ULONG_MAX)
>>> -		return -EINVAL;
>>> -
>>> -	region = kzalloc(sizeof(*region), GFP_KERNEL_ACCOUNT);
>>> -	if (!region)
>>> -		return -ENOMEM;
>>> -
>>> -	mutex_lock(&kvm->lock);
>>> -	region->pages = sev_pin_memory(kvm, range->addr, range->size, &region->npages, 1);
>>> -	if (IS_ERR(region->pages)) {
>>> -		ret = PTR_ERR(region->pages);
>>> -		mutex_unlock(&kvm->lock);
>>> -		goto e_free;
>>> -	}
>>> -
>>> -	region->uaddr = range->addr;
>>> -	region->size = range->size;
>>> -
>>> -	list_add_tail(&region->list, &sev->regions_list);
>>> -	mutex_unlock(&kvm->lock);
>>> -
>>> -	/*
>>> -	 * The guest may change the memory encryption attribute from C=0 -> C=1
>>> -	 * or vice versa for this memory range. Lets make sure caches are
>>> -	 * flushed to ensure that guest data gets written into memory with
>>> -	 * correct C-bit.
>>> -	 */
>>> -	sev_clflush_pages(region->pages, region->npages);
>>> -
>>> -	return ret;
>>> -
>>> -e_free:
>>> -	kfree(region);
>>> -	return ret;
>>> -}
>>> -
>>> -static struct enc_region *
>>> -find_enc_region(struct kvm *kvm, struct kvm_enc_region *range)
>>> -{
>>> -	struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
>>> -	struct list_head *head = &sev->regions_list;
>>> -	struct enc_region *i;
>>> -
>>> -	list_for_each_entry(i, head, list) {
>>> -		if (i->uaddr == range->addr &&
>>> -		    i->size == range->size)
>>> -			return i;
>>> -	}
>>> -
>>> -	return NULL;
>>> -}
>>> -
>>> -static void __unregister_enc_region_locked(struct kvm *kvm,
>>> -					   struct enc_region *region)
>>> -{
>>> -	sev_unpin_memory(kvm, region->pages, region->npages);
>>> -	list_del(&region->list);
>>> -	kfree(region);
>>> +	return 0;
>>>  }
>>>  
>>>  int svm_unregister_enc_region(struct kvm *kvm,
>>>  			      struct kvm_enc_region *range)
>>>  {
>>> -	struct enc_region *region;
>>> -	int ret;
>>> -
>>> -	/* If kvm is mirroring encryption context it isn't responsible for it */
>>> -	if (is_mirroring_enc_context(kvm))
>>> -		return -EINVAL;
>>> -
>>> -	mutex_lock(&kvm->lock);
>>> -
>>> -	if (!sev_guest(kvm)) {
>>> -		ret = -ENOTTY;
>>> -		goto failed;
>>> -	}
>>> -
>>> -	region = find_enc_region(kvm, range);
>>> -	if (!region) {
>>> -		ret = -EINVAL;
>>> -		goto failed;
>>> -	}
>>> -
>>> -	/*
>>> -	 * Ensure that all guest tagged cache entries are flushed before
>>> -	 * releasing the pages back to the system for use. CLFLUSH will
>>> -	 * not do this, so issue a WBINVD.
>>> -	 */
>>> -	wbinvd_on_all_cpus();
>>> -
>>> -	__unregister_enc_region_locked(kvm, region);
>>> -
>>> -	mutex_unlock(&kvm->lock);
>>>  	return 0;
>>> -
>>> -failed:
>>> -	mutex_unlock(&kvm->lock);
>>> -	return ret;
>>>  }
>>>  
>>>  int svm_vm_copy_asid_from(struct kvm *kvm, unsigned int source_fd)
>>> @@ -2018,7 +1904,6 @@ int svm_vm_copy_asid_from(struct kvm *kvm, unsigned int source_fd)
>>>  	mirror_sev->fd = source_sev->fd;
>>>  	mirror_sev->es_active = source_sev->es_active;
>>>  	mirror_sev->handle = source_sev->handle;
>>> -	INIT_LIST_HEAD(&mirror_sev->regions_list);
>>>  	ret = 0;
>>>  
>>>  	/*
>>> @@ -2038,8 +1923,6 @@ int svm_vm_copy_asid_from(struct kvm *kvm, unsigned int source_fd)
>>>  void sev_vm_destroy(struct kvm *kvm)
>>>  {
>>>  	struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
>>> -	struct list_head *head = &sev->regions_list;
>>> -	struct list_head *pos, *q;
>>>  
>>>  	WARN_ON(sev->num_mirrored_vms);
>>>  
>>> @@ -2066,18 +1949,6 @@ void sev_vm_destroy(struct kvm *kvm)
>>>  	 */
>>>  	wbinvd_on_all_cpus();
>>>  
>>> -	/*
>>> -	 * if userspace was terminated before unregistering the memory regions
>>> -	 * then lets unpin all the registered memory.
>>> -	 */
>>> -	if (!list_empty(head)) {
>>> -		list_for_each_safe(pos, q, head) {
>>> -			__unregister_enc_region_locked(kvm,
>>> -				list_entry(pos, struct enc_region, list));
>>> -			cond_resched();
>>> -		}
>>> -	}
>>> -
>>>  	sev_unbind_asid(kvm, sev->handle);
>>>  	sev_asid_free(sev);
>>>  }
>>> @@ -2946,13 +2817,90 @@ void sev_vcpu_deliver_sipi_vector(struct kvm_vcpu *vcpu, u8 vector)
>>>  	ghcb_set_sw_exit_info_2(svm->sev_es.ghcb, 1);
>>>  }
>>>  
>>> +void sev_pin_spte(struct kvm *kvm, gfn_t gfn, enum pg_level level,
>>> +		  kvm_pfn_t pfn)
>>> +{
>>> +	struct kvm_arch_memory_slot *aslot;
>>> +	struct kvm_memory_slot *slot;
>>> +	gfn_t rel_gfn, pin_pfn;
>>> +	unsigned long npages;
>>> +	kvm_pfn_t old_pfn;
>>> +	int i;
>>> +
>>> +	if (!sev_guest(kvm))
>>> +		return;
>>> +
>>> +	if (WARN_ON_ONCE(is_error_noslot_pfn(pfn) || kvm_is_reserved_pfn(pfn)))
>>> +		return;
>>> +
>>> +	/* Tested till 1GB pages */
>>> +	if (KVM_BUG_ON(level > PG_LEVEL_1G, kvm))
>>> +		return;
>>> +
>>> +	slot = gfn_to_memslot(kvm, gfn);
>>> +	if (!slot || !slot->arch.pfns)
>>> +		return;
>>> +
>>> +	/*
>>> +	 * Use relative gfn index within the memslot for the bitmap as well as
>>> +	 * the pfns array
>>> +	 */
>>> +	rel_gfn = gfn - slot->base_gfn;
>>> +	aslot = &slot->arch;
>>> +	pin_pfn = pfn;
>>> +	npages = KVM_PAGES_PER_HPAGE(level);
>>> +
>>> +	/* Pin the page, KVM doesn't yet support page migration. */
>>> +	for (i = 0; i < npages; i++, rel_gfn++, pin_pfn++) {
>>> +		if (test_bit(rel_gfn, aslot->pinned_bitmap)) {
>>> +			old_pfn = aslot->pfns[rel_gfn];
>>> +			if (old_pfn == pin_pfn)
>>> +				continue;
>>> +
>>> +			put_page(pfn_to_page(old_pfn));
>>> +		}
>>> +
>>> +		set_bit(rel_gfn, aslot->pinned_bitmap);
>>> +		aslot->pfns[rel_gfn] = pin_pfn;
>>> +		get_page(pfn_to_page(pin_pfn));
>>
>>
>> I assume this is to replace KVM_MEMORY_ENCRYPT_REG_REGION, which ends up
>> calling svm_register_enc_region()->sev_pin_memory(), correct?
> 
> Yes, that is correct.
>>
>> sev_pin_memory() correctly checks the RLIMIT_MEMLOCK and uses
>> pin_user_pages_fast().
>>
>> I have to strongly assume that sev_pin_memory() is *wrong* as is because
>> it's supposed to supply FOLL_LONGTERM -- after all we're pinning these
>> pages possibly forever.
>>
>>
>> I might be wrong but
>>
>> 1. You are missing the RLIMIT_MEMLOCK check
> 
> Yes, I will add this check during the enc_region registration.
> 
>> 2. get_page() is the wong way of long-term pinning a page. You would
>> have to mimic what pin_user_pages_fast(FOLL_LONGTERM) does to eventually
>> get it right (e.g., migrate the page off of MIGRATE_CMA or ZONE_MOVABLE).
> 
> Let me go through this and I will come back. Thanks for pointing this out.

I asusme the "issue" is that KVM uses mmu notifier and does a simple
get_user_pages() to obtain the references, to drop the reference when
the entry is invalidated via a mmu notifier call. So once you intent to
long-term pin, it's already to late.

If you could teach KVM to do a long-term pin when stumbling over these
special encrypted memory regions (requires a proper matching
unpin_user_pages() call from KVM), then you could "take over" that pin
by get_page(), and let KVM do the ordinary put_page(), while you would
do the unpin_user_pages().

-- 
Thanks,

David / dhildenb