On Fri, Jul 24, 2020 at 06:54:46PM -0500, eric van tassell wrote:
> Improve 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.
>
> Implement the code to do the pinning (sev_get_page) and the notifier
> sev_set_spte_notify().
>
> Track the pinned pages with xarray so they can be released during guest
> termination.
I like that SEV is trying to be a better citizen, but this is trading one
hack for another.
- KVM goes through a lot of effort to ensure page faults don't need to
allocate memory, and this throws all that effort out the window.
- Tracking all gfns in a separate database (from the MMU) is wasteful.
- Having to wait to free pinned memory until the VM is destroyed is less
than ideal.
More thoughts in the next patch.
> Signed-off-by: eric van tassell <Eric.VanTassell@xxxxxxx>
> ---
> arch/x86/kvm/svm/sev.c | 71 ++++++++++++++++++++++++++++++++++++++++++
> arch/x86/kvm/svm/svm.c | 2 ++
> arch/x86/kvm/svm/svm.h | 3 ++
> 3 files changed, 76 insertions(+)
>
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index f7f1f4ecf08e..040ae4aa7c5a 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -184,6 +184,8 @@ static int sev_guest_init(struct kvm *kvm, struct kvm_sev_cmd *argp)
> sev->asid = asid;
> INIT_LIST_HEAD(&sev->regions_list);
>
> + xa_init(&sev->pages_xarray);
> +
> return 0;
>
> e_free:
> @@ -415,6 +417,42 @@ static unsigned long get_num_contig_pages(unsigned long idx,
> return pages;
> }
>
> +static int sev_get_page(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn)
> +{
> + struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
> + struct xarray *xa = &sev->pages_xarray;
> + struct page *page = pfn_to_page(pfn);
> + int ret;
> +
> + /* store page at index = gfn */
> + ret = xa_insert(xa, gfn, page, GFP_ATOMIC);
> + if (ret == -EBUSY) {
> + /*
> + * If xa_insert returned -EBUSY, the gfn was already associated
> + * with a struct page *.
> + */
> + struct page *cur_page;
> +
> + cur_page = xa_load(xa, gfn);
> + /* If cur_page == page, no change is needed, so return 0 */
> + if (cur_page == page)
> + return 0;
> +
> + /* Release the page that was stored at index = gfn */
> + put_page(cur_page);
> +
> + /* Return result of attempting to store page at index = gfn */
> + ret = xa_err(xa_store(xa, gfn, page, GFP_ATOMIC));
> + }
> +
> + if (ret)
> + return ret;
> +
> + get_page(page);
> +
> + return 0;
> +}
> +
> static int sev_launch_update_data(struct kvm *kvm, struct kvm_sev_cmd *argp)
> {
> unsigned long vaddr, vaddr_end, next_vaddr, npages, pages, size, i;
> @@ -1085,6 +1123,8 @@ 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;
> + XA_STATE(xas, &sev->pages_xarray, 0);
> + struct page *xa_page;
>
> if (!sev_guest(kvm))
> return;
> @@ -1109,6 +1149,12 @@ void sev_vm_destroy(struct kvm *kvm)
> }
> }
>
> + /* Release each pinned page that SEV tracked in sev->pages_xarray. */
> + xas_for_each(&xas, xa_page, ULONG_MAX) {
> + put_page(xa_page);
> + }
> + xa_destroy(&sev->pages_xarray);
> +
> mutex_unlock(&kvm->lock);
>
> sev_unbind_asid(kvm, sev->handle);
> @@ -1193,3 +1239,28 @@ void pre_sev_run(struct vcpu_svm *svm, int cpu)
> svm->vmcb->control.tlb_ctl = TLB_CONTROL_FLUSH_ASID;
> vmcb_mark_dirty(svm->vmcb, VMCB_ASID);
> }
> +
> +int sev_set_spte_notify(struct kvm_vcpu *vcpu, gfn_t gfn, kvm_pfn_t pfn,
> + int level, bool mmio, u64 *spte)
> +{
> + int rc;
> +
> + if (!sev_guest(vcpu->kvm))
> + return 0;
> +
> + /* MMIO page contains the unencrypted data, no need to lock this page */
> + if (mmio)
Rather than make this a generic set_spte() notify hook, I think it makes
more sense to specifying have it be a "pin_spte" style hook. That way the
caller can skip mmio PFNs as well as flows that can't possibly be relevant
to SEV, e.g. the sync_page() flow.
> + return 0;
> +
> + rc = sev_get_page(vcpu->kvm, gfn, pfn);
> + if (rc)
> + return rc;
> +
> + /*
> + * Flush any cached lines of the page being added since "ownership" of
> + * it will be transferred from the host to an encrypted guest.
> + */
> + clflush_cache_range(__va(pfn << PAGE_SHIFT), page_level_size(level));
> +
> + return 0;
> +}
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 535ad311ad02..9b304c761a99 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -4130,6 +4130,8 @@ static struct kvm_x86_ops svm_x86_ops __initdata = {
> .need_emulation_on_page_fault = svm_need_emulation_on_page_fault,
>
> .apic_init_signal_blocked = svm_apic_init_signal_blocked,
> +
> + .set_spte_notify = sev_set_spte_notify,
> };
>
> static struct kvm_x86_init_ops svm_init_ops __initdata = {
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index 121b198b51e9..8a5c01516c89 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -65,6 +65,7 @@ struct kvm_sev_info {
> int fd; /* SEV device fd */
> unsigned long pages_locked; /* Number of pages locked */
> struct list_head regions_list; /* List of registered regions */
> + struct xarray pages_xarray; /* List of PFN locked */
> };
>
> struct kvm_svm {
> @@ -488,5 +489,7 @@ int svm_unregister_enc_region(struct kvm *kvm,
> void pre_sev_run(struct vcpu_svm *svm, int cpu);
> int __init sev_hardware_setup(void);
> void sev_hardware_teardown(void);
> +int sev_set_spte_notify(struct kvm_vcpu *vcpu, gfn_t gfn, kvm_pfn_t pfn,
> + int level, bool mmio, u64 *spte);
>
> #endif
> --
> 2.17.1
>
