On 11/23/22 09:37, Sean Christopherson wrote: > On Wed, Nov 23, 2022, Dave Hansen wrote: >> There's no way we can guarantee _that_. For one, the PAMT* allocations >> can always fail. I guess we could ask sysadmins to fire up a guest to >> "prime" things, but that seems a little silly. Maybe that would work as >> the initial implementation that we merge, but I suspect our users will >> demand more determinism, maybe a boot or module parameter. > Oh, you mean all of TDX initialization? I thought "initialization" here mean just > doing tdx_enable(). Yes, but the first call to tdx_enable() does TDH_SYS_INIT and all the subsequent work to get the module going. > Yeah, that's not going to be a viable option. Aside from lacking determinisim, > it would be all too easy to end up on a system with fragmented memory that can't > allocate the PAMTs post-boot. For now, the post-boot runtime PAMT allocations are the one any only way that TDX can be initialized. I pushed for it to be done this way. Here's why: Doing tdx_enable() is relatively slow and it eats up a non-zero amount of physically contiguous RAM for metadata (~1/256th or ~0.4% of RAM). Systems that support TDX but will never run TDX guests should not pay that cost. That means that we either make folks opt-in at boot-time or we try to make a best effort at runtime to do the metadata allocations. >From my perspective, the best-effort stuff is absolutely needed. Users are going to forget the command-line opt in and there's no harm in _trying_ the big allocations even if they fail. Second, in reality, the "real" systems that can run TDX guests are probably not going to sit around fragmenting memory for a month before they run their first guest. They're going to run one shortly after they boot when memory isn't fragmented and the best-effort allocation will work really well. Third, if anyone *REALLY* cared to make it reliable *and* wanted to sit around fragmenting memory for a month, they could just start a TDX guest and kill it to get TDX initialized. This isn't ideal. But, to me, it beats defining some new, separate ABI (or boot/module option) to do it. So, let's have those discussions. Long-term, what *is* the most reliable way to get the TDX module loaded with 100% determinism? What new ABI or interfaces are needed? Also, is that 100% determinism required the moment this series is merged? Or, can we work up to it? I think it can wait until this particular series is farther along.
