On Sat, Jan 07, 2023, Robert Hoo wrote: > On Sat, 2023-01-07 at 00:38 +0000, Sean Christopherson wrote: > > On Fri, Dec 09, 2022, Robert Hoo wrote: > > > If LAM enabled, CR4.LAM_SUP is owned by guest; otherwise, reserved. > > > > Why is it passed through to the guest? > > I think no need to intercept guest's control over CR4.LAM_SUP, which > controls LAM appliance to supervisor mode address. That's not a sufficient justification. KVM doesn't strictly need to intercept most CR4 bits, but not intercepting has performance implications, e.g. KVM needs to do a VMREAD(GUEST_CR4) to get LAM_SUP if the bit is pass through. As a base rule, KVM intercepts CR4 bits unless there's a reason not to, e.g. if the CR4 bit in question is written frequently by real guests and/or never consumed by KVM.
