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

Nikunj A. Dadhania
2022-01-28T11:04:58Z

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?

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