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

David Hildenbrand
2022-01-31T12:41:39Z

On 31.01.22 13:18, Nikunj A. Dadhania wrote:
> On 1/31/2022 5:26 PM, David Hildenbrand wrote:
>> On 28.01.22 12:08, David Hildenbrand wrote:
>>> On 28.01.22 12:04, Nikunj A. Dadhania wrote:
>>>> 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@amd.com>
>>>>>>>> ---
>>>>>>>> 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?
>>>>
>>>
>>> The issue is that pin_user_pages(FOLL_LONGTERM) might have to migrate
>>> the page, which will fail if there is already an additional reference
>>> from get_user_pages_*().
>>>
>>
>> Minor addition: hva_to_pfn_{slow,fast}() *don't* take a reference,
>
> hva_to_pfn_fast() does take a reference, not able to find in _slow() though.

Ah, my fault, you're correct and my memory is wrong.

>
> ->get_user_page_fast_only()
> -> get_user_pages_fast_only()
> ...
> gup_flags |= FOLL_GET | FOLL_FAST_ONLY;
> ...

__get_user_pages_locked() has

if (pages && !(flags & FOLL_PIN))
flags |= FOLL_GET;$

I could have sworn we'd have code to lookup a page without the need to
grab a reference for MMU notifier purposes in KVM's MMU.

But looking into the details, I think we simply get a reference, map the
page, and then release the reference.

--
Thanks,

David / dhildenb