On 03/07/20 04:50, Sean Christopherson wrote: > Introduce a new capability, KVM_CAP_MEMSLOT_ZAP_CONTROL, to allow > userspace to control the memslot zapping behavior on a per-VM basis. > x86's default behavior is to zap all SPTEs, including the root shadow > page, across all memslots. While effective, the nuke and pave approach > isn't exactly performant, especially for large VMs and/or VMs that > heavily utilize RO memslots for MMIO devices, e.g. option ROMs. > > On a vanilla VM with 6gb of RAM, the targeted zap reduces the number of > EPT violations during boot by ~14% with THP enabled in the host, and by > ~7% with THP disabled in the host. On a much more custom VM with 32gb > and a significant amount of memslot zapping, this can reduce the number > of EPT violations by 50% during guest boot, and improve boot time by > as much as 25%. > > Keep the current x86 memslot zapping behavior as the default, as there's > an unresolved bug that pops up when zapping only the affected memslot, > and the exact conditions that trigger the bug are not fully known. See > https://patchwork.kernel.org/patch/10798453 for details. > > Implement the capability as a set of flags so that other architectures > might be able to use the capability without having to conform to x86's > semantics. It's bad that we have no clue what's causing the bad behavior, but I don't think it's wise to have a bug that is known to happen when you enable the capability. :/ Paolo
