Date:   Wed, 26 Jan 2022 11:46:56 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Nikunj A Dadhania <nikunj@....com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Brijesh Singh <brijesh.singh@....com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Peter Gonda <pgonda@...gle.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 3/6] KVM: SVM: Implement demand page pinning

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@....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?

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

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).

-- 
Thanks,

David / dhildenb

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