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

Vitaly Kuznetsov
2020-05-27T08:39:42Z

Sean Christopherson <sean.j.christopherson@intel.com> writes:

> 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.

We have rdmsr_safe() for that! :-) MSR (and hypercall to that matter)
should have an associated CPUID feature bit of course.

Yes, hypercall + CPUID would do but normally we treat CPUID data as
static and in this case we'll make it a dynamically flipping
bit. Especially if we introduce 'KVM_HC_DISABLE_MEM_PROTECTED' later.

>
>> 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.
>

And maybe even help us to resolve 'reboot' problem.

--
Vitaly