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Date: Tue, 3 Jul 2018 22:41:14 +0200 (CEST)
From: Thomas Gleixner <tglx@...utronix.de>
To: Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
cc: x86@...nel.org, platform-driver-x86@...r.kernel.org,
dave.hansen@...el.com, sean.j.christopherson@...el.com,
nhorman@...hat.com, npmccallum@...hat.com,
linux-sgx@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
"H. Peter Anvin" <hpa@...or.com>,
"open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v12 09/13] x86/sgx: EPC page allocation routines
On Tue, 3 Jul 2018, Jarkko Sakkinen wrote:
>
> +#define SGX_NR_TO_SCAN 16
> +#define SGX_NR_LOW_PAGES 32
> +#define SGX_NR_HIGH_PAGES 64
> +
> bool sgx_enabled __ro_after_init;
> EXPORT_SYMBOL(sgx_enabled);
> bool sgx_lc_enabled __ro_after_init;
> EXPORT_SYMBOL(sgx_lc_enabled);
> +LIST_HEAD(sgx_active_page_list);
> +EXPORT_SYMBOL(sgx_active_page_list);
> +DEFINE_SPINLOCK(sgx_active_page_list_lock);
> +EXPORT_SYMBOL(sgx_active_page_list_lock);
Why is all of this exported. If done right then no call site has to fiddle
with the list and the lock at all.
> static atomic_t sgx_nr_free_pages = ATOMIC_INIT(0);
> static struct sgx_epc_bank sgx_epc_banks[SGX_MAX_EPC_BANKS];
> static int sgx_nr_epc_banks;
> +static struct task_struct *ksgxswapd_tsk;
> +static DECLARE_WAIT_QUEUE_HEAD(ksgxswapd_waitq);
> +
> +static void sgx_swap_cluster(void)
> +{
> + struct sgx_epc_page *cluster[SGX_NR_TO_SCAN + 1];
> + struct sgx_epc_page *epc_page;
> + int i;
> + int j;
int i, j;
> + memset(cluster, 0, sizeof(cluster));
> +
> + for (i = 0, j = 0; i < SGX_NR_TO_SCAN; i++) {
> + spin_lock(&sgx_active_page_list_lock);
> + if (list_empty(&sgx_active_page_list)) {
> + spin_unlock(&sgx_active_page_list_lock);
> + break;
> + }
> + epc_page = list_first_entry(&sgx_active_page_list,
> + struct sgx_epc_page, list);
> + if (!epc_page->impl->ops->get(epc_page)) {
> + list_move_tail(&epc_page->list, &sgx_active_page_list);
> + spin_unlock(&sgx_active_page_list_lock);
> + continue;
> + }
> + list_del(&epc_page->list);
> + spin_unlock(&sgx_active_page_list_lock);
> +
> + if (epc_page->impl->ops->reclaim(epc_page)) {
> + cluster[j++] = epc_page;
> + } else {
> + spin_lock(&sgx_active_page_list_lock);
> + list_add_tail(&epc_page->list, &sgx_active_page_list);
> + spin_unlock(&sgx_active_page_list_lock);
> + epc_page->impl->ops->put(epc_page);
> + }
> + }
> +
> + for (i = 0; cluster[i]; i++) {
> + epc_page = cluster[i];
> + epc_page->impl->ops->block(epc_page);
> + }
> +
> + for (i = 0; cluster[i]; i++) {
> + epc_page = cluster[i];
> + epc_page->impl->ops->write(epc_page);
> + epc_page->impl->ops->put(epc_page);
> + sgx_free_page(epc_page);
> + }
Thanks a lot for commenting this piece of art thoughtfully. It's entirely
clear how all of this works now.
> +}
> +
> +static int ksgxswapd(void *p)
> +{
> + set_freezable();
> +
> + while (!kthread_should_stop()) {
> + if (try_to_freeze())
> + continue;
> +
> + wait_event_freezable(ksgxswapd_waitq, kthread_should_stop() ||
> + atomic_read(&sgx_nr_free_pages) <
> + SGX_NR_HIGH_PAGES);
> +
> + if (atomic_read(&sgx_nr_free_pages) < SGX_NR_HIGH_PAGES)
> + sgx_swap_cluster();
> + }
> +
> + pr_info("%s: done\n", __func__);
Really useful.
> + return 0;
> +}
> +
> +static struct sgx_epc_page *sgx_try_alloc_page(struct sgx_epc_page_impl *impl)
> +{
> + struct sgx_epc_bank *bank;
> + struct sgx_epc_page *page = NULL;
> + int i;
> +
> + for (i = 0; i < sgx_nr_epc_banks; i++) {
> + bank = &sgx_epc_banks[i];
> +
> + down_write(&bank->lock);
> +
> + if (atomic_read(&bank->free_cnt))
And these atomics are required becasue bank->lock protection is not
sufficient or what am I missing here?
> + page = bank->pages[atomic_dec_return(&bank->free_cnt)];
> +
> + up_write(&bank->lock);
> +
> + if (page)
> + break;
> + }
> +
> + if (page) {
> + atomic_dec(&sgx_nr_free_pages);
> + page->impl = impl;
> + }
> +
> + return page;
> +}
> +
> +/**
> + * sgx_alloc_page - allocate an EPC page
> + * @flags: allocation flags
> + * @impl: implementation for the struct sgx_epc_page
> + *
> + * Try to grab a page from the free EPC page list. If there is a free page
> + * available, it is returned to the caller. If called with SGX_ALLOC_ATOMIC,
> + * the function will return immediately if the list is empty. Otherwise, it
> + * will swap pages up until there is a free page available. Upon returning the
> + * low watermark is checked and ksgxswapd is waken up if we are below it.
> + *
> + * Return:
> + * a &struct sgx_epc_page instace,
> + * -ENOMEM if all pages are unreclaimable,
> + * -EBUSY when called with SGX_ALLOC_ATOMIC and out of free pages
> + */
> +struct sgx_epc_page *sgx_alloc_page(struct sgx_epc_page_impl *impl,
> + unsigned int flags)
> +{
> + struct sgx_epc_page *entry;
> +
> + for ( ; ; ) {
> + entry = sgx_try_alloc_page(impl);
> + if (entry)
> + break;
> +
> + if (list_empty(&sgx_active_page_list))
> + return ERR_PTR(-ENOMEM);
> +
> + if (flags & SGX_ALLOC_ATOMIC) {
> + entry = ERR_PTR(-EBUSY);
> + break;
> + }
> +
> + if (signal_pending(current)) {
> + entry = ERR_PTR(-ERESTARTSYS);
> + break;
> + }
> +
> + sgx_swap_cluster();
> + schedule();
> + }
> +
> + if (atomic_read(&sgx_nr_free_pages) < SGX_NR_LOW_PAGES)
> + wake_up(&ksgxswapd_waitq);
What's the logic of SGX_NR_LOW_PAGES vs. SGX_NR_HIGH_PAGES?
> +
> + return entry;
> +}
> +EXPORT_SYMBOL(sgx_alloc_page);
> +
> +/**
> + * sgx_free_page - free an EPC page
> + *
> + * @page: any EPC page
> + *
> + * Remove an EPC page and insert it back to the list of free pages.
> + *
> + * Return: SGX error code
> + */
> +int sgx_free_page(struct sgx_epc_page *page)
> +{
> + struct sgx_epc_bank *bank = SGX_EPC_BANK(page);
> + int ret;
> +
> + ret = sgx_eremove(page);
> + if (ret) {
> + pr_debug("EREMOVE returned %d\n", ret);
> + return ret;
> + }
> +
> + down_read(&bank->lock);
> + bank->pages[atomic_inc_return(&bank->free_cnt) - 1] = page;
> + atomic_inc(&sgx_nr_free_pages);
> + up_read(&bank->lock);
I have hard time to see the benefit of this reader/writer semaphore
here. Both sides which fiddle with the bank pages are doing a simple
de/increment of free_cnt and a store resp. load. So what justifies the
overhead of a rwsem?
> static __init int sgx_init_epc_bank(unsigned long addr, unsigned long size,
> unsigned long index,
> struct sgx_epc_bank *bank)
> @@ -114,6 +318,11 @@ static __init void sgx_page_cache_teardown(void)
> kfree(bank->pages);
> kfree(bank->pages_data);
> }
> +
> + if (ksgxswapd_tsk) {
> + kthread_stop(ksgxswapd_tsk);
> + ksgxswapd_tsk = NULL;
This stops the thread _AFTER_ freeing all the bank memory. Is that actually
correct?
Thanks,
tglx
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