Re: [RFC 02/16] x86/kvm: Introduce KVM memory protection feature

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Mon, May 25, 2020 at 06:15:25PM +0300, Kirill A. Shutemov wrote:
> On Mon, May 25, 2020 at 04:58:51PM +0200, Vitaly Kuznetsov wrote:
> > > @@ -727,6 +734,15 @@ static void __init kvm_init_platform(void)
> > >  {
> > >  	kvmclock_init();
> > >  	x86_platform.apic_post_init = kvm_apic_init;
> > > +
> > > +	if (kvm_para_has_feature(KVM_FEATURE_MEM_PROTECTED)) {
> > > +		if (kvm_hypercall0(KVM_HC_ENABLE_MEM_PROTECTED)) {
> > > +			pr_err("Failed to enable KVM memory protection\n");
> > > +			return;
> > > +		}
> > > +
> > > +		mem_protected = true;
> > > +	}
> > >  }
> > 
> > Personally, I'd prefer to do this via setting a bit in a KVM-specific
> > MSR instead. The benefit is that the guest doesn't need to remember if
> > it enabled the feature or not, it can always read the config msr. May
> > come handy for e.g. kexec/kdump.
> 
> I think we would need to remember it anyway. Accessing MSR is somewhat
> expensive. But, okay, I can rework it MSR if needed.

I think Vitaly is talking about the case where the kernel can't easily get
at its cached state, e.g. after booting into a new kernel.  The kernel would
still have an X86_FEATURE bit or whatever, providing a virtual MSR would be
purely for rare slow paths.

That being said, a hypercall plus CPUID bit might be better, e.g. that'd
allow the guest to query the state without risking a #GP.

> Note, that we can avoid the enabling algother, if we modify BIOS to deal
> with private/shared memory. Currently BIOS get system crash if we enable
> the feature from time zero.

Which would mesh better with a CPUID feature bit.