On 29/01/2018 09:46, David Woodhouse wrote: > I'd actually quite like to repeat the benchmark on the new fixed > microcode, if anyone has it yet, to see if that read/swap slowness is > still quite as excessive. I'm certainly not ruling this out, but I'm > just a little wary of premature optimisation, and I'd like to make sure > we have everything *else* in the KVM patches right first. > > The fact that the save-and-restrict macros I have in the tip of my > working tree at the moment are horrid and causing 0-day nastygrams, > probably doesn't help persuade me to favour the approach ;) > > ... hm, the CPU actually has separate MSR save/restore lists for > entry/exit, doesn't it? Is there any way to sanely make use of that and > do the restoration manually on vmentry but let it be automatic on > vmexit, by having it *only* in the guest's MSR-store area to be saved > on exit and restored on exit, but *not* in the host's MSR-store area? Right now we don't even use the store-on-vmexit list at all, so the Simplest Patch That Can Possibly Work is definitely the one using rdmsr/wrmsr. It's not really premature optimization---though it doesn't hurt that it isn't awfully slow. Paolo
