Re: [PATCH v19 038/130] KVM: TDX: create/destroy VM structure

[Isaku Yamahata]

On Fri, Mar 22, 2024 at 01:32:15PM +0800,
Yuan Yao <yuan.yao@xxxxxxxxxxxxxxx> wrote:

> On Fri, Mar 22, 2024 at 11:46:41AM +0800, Yuan Yao wrote:
> > On Thu, Mar 21, 2024 at 07:17:09AM -0700, Isaku Yamahata wrote:
> > > On Wed, Mar 20, 2024 at 01:12:01PM +0800,
> > > Chao Gao <chao.gao@xxxxxxxxx> wrote:
> ...
> > > > >+static int __tdx_td_init(struct kvm *kvm)
> > > > >+{
> > > > >+	struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
> > > > >+	cpumask_var_t packages;
> > > > >+	unsigned long *tdcs_pa = NULL;
> > > > >+	unsigned long tdr_pa = 0;
> > > > >+	unsigned long va;
> > > > >+	int ret, i;
> > > > >+	u64 err;
> > > > >+
> > > > >+	ret = tdx_guest_keyid_alloc();
> > > > >+	if (ret < 0)
> > > > >+		return ret;
> > > > >+	kvm_tdx->hkid = ret;
> > > > >+
> > > > >+	va = __get_free_page(GFP_KERNEL_ACCOUNT);
> > > > >+	if (!va)
> > > > >+		goto free_hkid;
> > > > >+	tdr_pa = __pa(va);
> > > > >+
> > > > >+	tdcs_pa = kcalloc(tdx_info->nr_tdcs_pages, sizeof(*kvm_tdx->tdcs_pa),
> > > > >+			  GFP_KERNEL_ACCOUNT | __GFP_ZERO);
> > > > >+	if (!tdcs_pa)
> > > > >+		goto free_tdr;
> > > > >+	for (i = 0; i < tdx_info->nr_tdcs_pages; i++) {
> > > > >+		va = __get_free_page(GFP_KERNEL_ACCOUNT);
> > > > >+		if (!va)
> > > > >+			goto free_tdcs;
> > > > >+		tdcs_pa[i] = __pa(va);
> > > > >+	}
> > > > >+
> > > > >+	if (!zalloc_cpumask_var(&packages, GFP_KERNEL)) {
> > > > >+		ret = -ENOMEM;
> > > > >+		goto free_tdcs;
> > > > >+	}
> > > > >+	cpus_read_lock();
> > > > >+	/*
> > > > >+	 * Need at least one CPU of the package to be online in order to
> > > > >+	 * program all packages for host key id.  Check it.
> > > > >+	 */
> > > > >+	for_each_present_cpu(i)
> > > > >+		cpumask_set_cpu(topology_physical_package_id(i), packages);
> > > > >+	for_each_online_cpu(i)
> > > > >+		cpumask_clear_cpu(topology_physical_package_id(i), packages);
> > > > >+	if (!cpumask_empty(packages)) {
> > > > >+		ret = -EIO;
> > > > >+		/*
> > > > >+		 * Because it's hard for human operator to figure out the
> > > > >+		 * reason, warn it.
> > > > >+		 */
> > > > >+#define MSG_ALLPKG	"All packages need to have online CPU to create TD. Online CPU and retry.\n"
> > > > >+		pr_warn_ratelimited(MSG_ALLPKG);
> > > > >+		goto free_packages;
> > > > >+	}
> > > > >+
> > > > >+	/*
> > > > >+	 * Acquire global lock to avoid TDX_OPERAND_BUSY:
> > > > >+	 * TDH.MNG.CREATE and other APIs try to lock the global Key Owner
> > > > >+	 * Table (KOT) to track the assigned TDX private HKID.  It doesn't spin
> > > > >+	 * to acquire the lock, returns TDX_OPERAND_BUSY instead, and let the
> > > > >+	 * caller to handle the contention.  This is because of time limitation
> > > > >+	 * usable inside the TDX module and OS/VMM knows better about process
> > > > >+	 * scheduling.
> > > > >+	 *
> > > > >+	 * APIs to acquire the lock of KOT:
> > > > >+	 * TDH.MNG.CREATE, TDH.MNG.KEY.FREEID, TDH.MNG.VPFLUSHDONE, and
> > > > >+	 * TDH.PHYMEM.CACHE.WB.
> > > > >+	 */
> > > > >+	mutex_lock(&tdx_lock);
> > > > >+	err = tdh_mng_create(tdr_pa, kvm_tdx->hkid);
> > > > >+	mutex_unlock(&tdx_lock);
> > > > >+	if (err == TDX_RND_NO_ENTROPY) {
> > > > >+		ret = -EAGAIN;
> > > > >+		goto free_packages;
> > > > >+	}
> > > > >+	if (WARN_ON_ONCE(err)) {
> > > > >+		pr_tdx_error(TDH_MNG_CREATE, err, NULL);
> > > > >+		ret = -EIO;
> > > > >+		goto free_packages;
> > > > >+	}
> > > > >+	kvm_tdx->tdr_pa = tdr_pa;
> > > > >+
> > > > >+	for_each_online_cpu(i) {
> > > > >+		int pkg = topology_physical_package_id(i);
> > > > >+
> > > > >+		if (cpumask_test_and_set_cpu(pkg, packages))
> > > > >+			continue;
> > > > >+
> > > > >+		/*
> > > > >+		 * Program the memory controller in the package with an
> > > > >+		 * encryption key associated to a TDX private host key id
> > > > >+		 * assigned to this TDR.  Concurrent operations on same memory
> > > > >+		 * controller results in TDX_OPERAND_BUSY.  Avoid this race by
> > > > >+		 * mutex.
> > > > >+		 */
> > > > >+		mutex_lock(&tdx_mng_key_config_lock[pkg]);
> > > >
> > > > the lock is superfluous to me. with cpu lock held, even if multiple CPUs try to
> > > > create TDs, the same set of CPUs (the first online CPU of each package) will be
> > > > selected to configure the key because of the cpumask_test_and_set_cpu() above.
> > > > it means, we never have two CPUs in the same socket trying to program the key,
> > > > i.e., no concurrent calls.
> > >
> > > Makes sense. Will drop the lock.
> >
> > Not get the point, the variable "packages" on stack, and it's
> > possible that "i" is same for 2 threads which are trying to create td.
> > Anything I missed ?
> 
> Got the point after synced with chao.
> in case of using for_each_online_cpu() it's safe to remove the mutex_lock(&tdx_mng_key_config_lock[pkg]),
> since every thread will select only 1 cpu for each sockets in same order, and requests submited
> to same cpu by smp_call_on_cpu() are ordered on the target cpu. That means removing the lock works for
> using for_each_online_cpu() but does NOT work for randomly pick up a cpu per socket.
> 
> Maybe it's just my issue that doesn't realize what's going on here, but
> I think it still worth to give comment here for why it works/does not work.

It's deserves comment. Will add it.
-- 
Isaku Yamahata <isaku.yamahata@xxxxxxxxx>