RE: [PATCH 04/14] KVM: VMX: Extend VMX controls macro shenanigans

[Vitaly Kuznetsov]

"Dong, Eddie" <eddie.dong@xxxxxxxxx> writes:

>> -----Original Message-----
>> From: Sean Christopherson <seanjc@xxxxxxxxxx>
>> Sent: Monday, June 27, 2022 6:38 PM
>> To: Dong, Eddie <eddie.dong@xxxxxxxxx>
>> Cc: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>; kvm@xxxxxxxxxxxxxxx; Paolo
>> Bonzini <pbonzini@xxxxxxxxxx>; Anirudh Rayabharam
>> <anrayabh@xxxxxxxxxxxxxxxxxxx>; Wanpeng Li <wanpengli@xxxxxxxxxxx>;
>> Jim Mattson <jmattson@xxxxxxxxxx>; Maxim Levitsky
>> <mlevitsk@xxxxxxxxxx>; linux-hyperv@xxxxxxxxxxxxxxx; linux-
>> kernel@xxxxxxxxxxxxxxx
>> Subject: Re: [PATCH 04/14] KVM: VMX: Extend VMX controls macro
>> shenanigans
>> 
>> On Mon, Jun 27, 2022, Dong, Eddie wrote:
>> > >  static inline void lname##_controls_clearbit(struct vcpu_vmx *vmx, u##bits
>> > > val)	\
>> > >  {
>> > > 	\
>> > > +	BUILD_BUG_ON(!(val & (KVM_REQ_VMX_##uname |
>> > > KVM_OPT_VMX_##uname)));	\
>> > >  	lname##_controls_set(vmx, lname##_controls_get(vmx) & ~val);
>> > > 	\
>> > >  }
>> >
>> > With this, will it be safer if we present L1 CTRL MSRs with the bits
>> > KVM really uses? Do I miss something?
>> 
>> KVM will still allow L1 to use features/controls that KVM itself doesn't use, but
>> exposing features/controls that KVM doesn't use will require a more explicit
>> "override" of sorts, e.g. to prevent advertising features that are supported in
>> hardware, known to KVM, but disabled for whatever reason, e.g. a CPU bug,
>> eVMCS incompatibility, module param, etc...
> Mmm, that is fine too.
> But, do we consider the potential need of migration for a L1 VMM ?
> Normally the VM can be configured to be as hardware neutral for better
> compatibility, or exposing as close to hardware feature as possible
> for performance.
> For nested features, I thought we didn't support migration if L1 VMM
> yet, so exposing hardware capability by default is fine at moment. We
> may revisit one day in future if we need to support migration.

Not sure I got your point, nested state migration is fully supported in
KVM. When migrating a guest, KVM makes sure the list of features exposed
in VMX control MSRs remain the same. This may not be the case if you use
something like "-cpu host" in QEMU but the problems are not specific to
nesting.

>  This MACRO do help anyway 😊
>
>> 
>> The intent of this BUILD_BUG_ON() is to detect KVM usage of bits that aren't
>> enabled by default, i.e. to lower the probability that a control gets used by KVM
>> but isn't exposed to L1 because it's a dynamically enabled control.

-- 
Vitaly