Re: [PATCH v3 2/9] KVM: x86: Add CR4.LAM_SUP in guest owned bits

[Sean Christopherson <seanjc@google.com>]

On Tue, Jan 10, 2023, Robert Hoo wrote:
> On Mon, 2023-01-09 at 16:29 +0000, Sean Christopherson wrote:
> > 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.
> 
> From these 2 points to judge:
> CR4.LAM_SUP is written frequently by guest? I'm not sure, as native
> kernel enabling patch has LAM_U57 only yet, not sure its control will
> be per-process/thread or whole kernel-level. If it its use case is
> kasan kind of, would you expect it will be frequently guest written?

Controlling a kernel-level knob on a per-process basis would be bizarre.  But
the expected use case definitely needs to be understood.  I assume Kirill, or
whoever is doing the LAM_SUP implementation, can provide answers.

> Never consumed by KVM? false, e.g. kvm_untagged_addr() will read this
> bit. But not frequently, I think, at least by this patch set.

Untagging an address will need to be done any time KVM consumes a guest virtual
address, i.e. performs any kind of emulation.  That's not super high frequency
on modern CPUs, but it's not exactly rare either.

> So in general, you suggestion/preference? I'm all right on both
> choices.

Unless guests will be touching CR4.LAM_SUP on context switches, intercepting is
unquestionably the right choice.