On Fri, Aug 29, 2025 at 05:33:48AM +0800, Edgecombe, Rick P wrote: > On Thu, 2025-08-28 at 13:26 -0700, Sean Christopherson wrote: > > Me confused. This is pre-boot, not the normal fault path, i.e. blocking other > > operations is not a concern. > > Just was my recollection of the discussion. I found it: > https://lore.kernel.org/lkml/Zbrj5WKVgMsUFDtb@xxxxxxxxxx/ > > > > > If tdh_mr_extend() is too heavy for a non-preemptible section, then the current > > code is also broken in the sense that there are no cond_resched() calls. The > > vast majority of TDX hosts will be using non-preemptible kernels, so without an > > explicit cond_resched(), there's no practical difference between extending the > > measurement under mmu_lock versus outside of mmu_lock. > > > > _If_ we need/want to do tdh_mr_extend() outside of mmu_lock, we can and should > > still do tdh_mem_page_add() under mmu_lock. > > I just did a quick test and we should be on the order of <1 ms per page for the > full loop. I can try to get some more formal test data if it matters. But that > doesn't sound too horrible? > > tdh_mr_extend() outside MMU lock is tempting because it doesn't *need* to be > inside it. But maybe a better reason is that we could better handle errors > outside the fault. (i.e. no 5 line comment about why not to return an error in > tdx_mem_page_add() due to code in another file). > > I wonder if Yan can give an analysis of any zapping races if we do that. I actually proposed to have write mmu_lock around tdh_mem_page_add() and tdh_mr_extend(), as in https://lore.kernel.org/kvm/Ztfn5gh5888PmEIe@xxxxxxxxxxxxxxxxxxxxxxxxx. I don't see any reason why tdh_mr_extend() can't be done inside mmu_lock in the pre-boot stage. But the previous conclusion was that with slots_lock and filemap invalidation lock, it's ok to invoke tdh_mem_page_add() and tdh_mr_extend() without any mmu_lock. The nr_premapped can also detect the unexpected zap.
