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

Nikunj A. Dadhania
2022-01-28T06:58:07Z

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.

Regards
Nikunj