On Sun, Jan 08, 2023 at 03:09:44PM -0500, Dave Voutila wrote: > > Philip Guenther <guent...@gmail.com> writes: > > > On Sat, Jan 7, 2023 at 11:04 AM Dave Voutila <d...@sisu.io> wrote: > > > > Bringing this to tech@ to increase my chance of someone testing my > > diff. > > > > As reported in this thread on misc@ [1], I believe newer Intel hardware > > may be experiencing issues hosting Linux guests under vmm/vmd. It looks > > like there are some newer instructions Intel added (TPAUSE specifically) > > that also involve some new MSR(s). > > > > I don't have 12th gen Intel hardware to test this on (I think that's > > Alder Lake). I'd like to mask this feature from vmm guests since it's > > related to an MSR we don't yet pass through or emulate and has to do > > with the TSC (which has it's own challenges in vmm). > > > > For someone testing, you should be able to grab an Alpine Linux iso > > (-virt flavor) and boot it with vmd with the diff. (Without it should > > "hang" and spike CPU or just die.) Also check that WAITPKG shows up in > > your dmesg on the cpu feature output. > > > > This seem like it'll obviously work, but I guess it seems to me that this > > "opt-out" approach is generally > > unsafe/unstable and vmd should consider actively switching to "opt-in" on > > all these CPUID feature bits. I mean, > > what bits are defined in the SEFF first-leaf EDX that _do_ work with vmd? > > > > Great point (I think you mean ECX). Here's an updated diff that flips it > to a whitelist so Intel/AMD don't burn me with these new bits in the > future. This better?
I don't mean to fearmonger, but doesn't this open vmm(4) and the wider kernel up to risk from all future "unknown unknowns"? We're basically saying the guest can use any feature passed through from the host before any developer has even glanced at the documentation for the new bit, right? I'm not saying the "opt-out" approach shown in your patch is even *likely* to be a problem. But the "opt-in" approach doesn't seem unreasonably conservative to me. When something is widely used enough to cause a problem, we add the bit. Or maybe every few months we ought to check for a new x86-64 ISA pdf from Intel, since they seem to be the ones expanding the feature set.
