Re: [PATCH v4 14/19] KVM: SVM: Check that the current CPU supports SVM in kvm_is_svm_supported()

[Sean Christopherson <seanjc@google.com>]

On Mon, Jul 24, 2023, Peter Zijlstra wrote:
> On Fri, Jul 21, 2023 at 01:18:54PM -0700, Sean Christopherson wrote:
> > Check "this" CPU instead of the boot CPU when querying SVM support so that
> > the per-CPU checks done during hardware enabling actually function as
> > intended, i.e. will detect issues where SVM isn't support on all CPUs.
> 
> Is that a realistic concern?

It's not a concern in the sense that it should never happen, but I know of at
least one example where VMX on Intel completely disappeared[1].  The "compatibility"
checks are really more about the entire VMX/SVM feature set, the base VMX/SVM
support check is just an easy and obvious precursor to the full compatibility
checks.

Of course, SVM doesn't currently have compatibility checks on the full SVM feature
set, but that's more due to lack of a forcing function than a desire to _not_ have
them.  Intel CPUs have a pesky habit of bugs, ucode updates, and/or in-field errors
resulting in VMX features randomly appearing or disappearing.  E.g. there's an
ongoing buzilla (sorry) issue[2] where a user is only able to load KVM *after* a
suspend+resume cycle, because TSC scaling only shows up on one socket immediately
after boot, which is then somehow resolved by suspend+resume.

[1] 009bce1df0bb ("x86/split_lock: Don't write MSR_TEST_CTRL on CPUs that aren't whitelisted")
[2] https://bugzilla.kernel.org/show_bug.cgi?id=217574