Re: x86/bugs: KVM: Add support for SRSO_MSR_FIX, back for moar

Borislav Petkov
2025-05-01T08:19:44Z

On Wed, Apr 30, 2025 at 04:33:19PM -0700, Sean Christopherson wrote:
> Eww. That's quite painful, and completely disallowing enable_virt_on_load is
> undesirable, e.g. for use cases where the host is (almost) exclusively running
> VMs.

I wanted to stay generic... :-)

> Best idea I have is to throw in the towel on getting fancy, and just maintain a
> dedicated count in SVM.
>
> Alternatively, we could plumb an arch hook into kvm_create_vm() and kvm_destroy_vm()
> that's called when KVM adds/deletes a VM from vm_list, and key off vm_list being
> empty. But that adds a lot of boilerplate just to avoid a mutex+count.

FWIW, that was Tom's idea.

> +static void svm_srso_add_remove_vm(int count)
> +{
> + bool set;
> +
> + if (!cpu_feature_enabled(X86_FEATURE_SRSO_BP_SPEC_REDUCE))
> + return;
> +
> + guard(mutex)(&srso_lock);
> +
> + set = !srso_nr_vms;
> + srso_nr_vms += count;
> +
> + WARN_ON_ONCE(srso_nr_vms < 0);
> + if (!set && srso_nr_vms)
> + return;

So instead of doing this "by-foot", I would've used any of those
atomic_inc_return() and atomic_dec_and_test() and act upon the value when it
becomes 0 or !0 instead of passing 1 and -1. Because the count is kinda
implicit...

But yeah, not a biggie - that solves the issue too.

Thanks!

--
Regards/Gruss,
Boris.