On Thu, 2023-01-12 at 08:56 +0800, Huang, Ying wrote: > "Huang, Kai" <kai.huang@xxxxxxxxx> writes: > > > On Tue, 2023-01-10 at 08:18 -0800, Hansen, Dave wrote: > > > On 1/10/23 04:09, Huang, Kai wrote: > > > > On Mon, 2023-01-09 at 08:51 -0800, Dave Hansen wrote: > > > > > On 1/9/23 03:48, Huang, Kai wrote: > > > > > > > > > > This can also be enhanced in the future, i.e. by allowing adding non-TDX > > > > > > > > > > memory to a separate NUMA node. In this case, the "TDX-capable" nodes > > > > > > > > > > and the "non-TDX-capable" nodes can co-exist, but the kernel/userspace > > > > > > > > > > needs to guarantee memory pages for TDX guests are always allocated from > > > > > > > > > > the "TDX-capable" nodes. > > > > > > > > > > > > > > > > Why does it need to be enhanced? What's the problem? > > > > > > > > > > > > The problem is after TDX module initialization, no more memory can be hot-added > > > > > > to the page allocator. > > > > > > > > > > > > Kirill suggested this may not be ideal. With the existing NUMA ABIs we can > > > > > > actually have both TDX-capable and non-TDX-capable NUMA nodes online. We can > > > > > > bind TDX workloads to TDX-capable nodes while other non-TDX workloads can > > > > > > utilize all memory. > > > > > > > > > > > > But probably it is not necessarily to call out in the changelog? > > > > > > > > > > Let's say that we add this TDX-compatible-node ABI in the future. What > > > > > will old code do that doesn't know about this ABI? > > > > > > > > Right. The old app will break w/o knowing the new ABI. One resolution, I > > > > think, is we don't introduce new userspace ABI, but hide "TDX-capable" and "non- > > > > TDX-capable" nodes in the kernel, and let kernel to enforce always allocating > > > > TDX guest memory from those "TDX-capable" nodes. > > > > > > That doesn't actually hide all of the behavior from users. Let's say > > > they do: > > > > > > numactl --membind=6 qemu-kvm ... > > > > > > In other words, take all of this guest's memory and put it on node 6. > > > There lots of free memory on node 6 which is TDX-*IN*compatible. Then, > > > they make it a TDX guest: > > > > > > numactl --membind=6 qemu-kvm -tdx ... > > > > > > What happens? Does the kernel silently ignore the --membind=6? Or does > > > it return -ENOMEM somewhere and confuse the user who has *LOTS* of free > > > memory on node 6. > > > > > > In other words, I don't think the kernel can just enforce this > > > internally and hide it from userspace. > > > > IIUC, the kernel, for instance KVM who has knowledge the 'task_struct' is a TDX > > guest, can manually AND "TDX-capable" node masks to task's mempolicy, so that > > the memory will always be allocated from those "TDX-capable" nodes. KVM can > > refuse to create the TDX guest if it found task's mempolicy doesn't have any > > "TDX-capable" node, and print out a clear message to the userspace. > > > > But I am new to the core-mm, so I might have some misunderstanding. > > KVM here means in-kernel KVM module? If so, KVM can only output some > message in dmesg. Which isn't very good for users to digest. It's > better for the user space QEMU to detect whether current configuration > is usable and respond to users, via GUI, or syslog, etc. I am not against this. For instance, maybe we can add some dedicated error code and let KVM return it to Qemu, but I don't want to speak for KVM guys. We can discuss this more when we have patches actually sent out to the community.
