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

[Nikunj A. Dadhania]

On 1/28/2022 1:57 PM, David Hildenbrand wrote:
> 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().
> 

The fault path looks like this in KVM x86 mmu code:

direct_page_fault()
-> kvm_faultin_pfn()
   -> __gfn_to_pfn_memslot()
      -> hva_to_pfn()
         -> hva_to_pfn_{slow,fast}()
            -> get_user_pages_*()      <<<<==== This is where the
                                                reference is taken

Next step is to create the mappings which is done in below functions:

-> kvm_tdp_mmu_map() / __direct_map()

   -> Within this function (patch 1/6), I call sev_pin_spte to take an extra 
      reference to pin it using get_page. 

      Is it possible to use pin_user_pages(FOLL_LONGTERM) here? Wouldn't that 
      be equivalent to "take over" solution that you are suggesting?

Reference is released when direct_page_fault() completes using put_page()

Later when the SEV VM is shutting down, I can do unpin_user_pages() for the 
pinned pages.

Regards
Nikunj