Re: [PATCH 1/6] KVM: x86: Virtualize CR4.LASS

[Chao Gao]

On Tue, Apr 25, 2023 at 10:35:25AM +0800, Zeng Guang wrote:
>
>On 4/24/2023 3:32 PM, Gao, Chao wrote:
>> On Thu, Apr 20, 2023 at 09:37:19PM +0800, Zeng Guang wrote:
>> > Virtualize CR4.LASS[bit 27] under KVM control instead of being guest-owned
>> > as CR4.LASS generally set once for each vCPU at boot time and won't be
>> > toggled at runtime. Besides, only if VM has LASS capability enumerated with
>> > CPUID.(EAX=07H.ECX=1):EAX.LASS[bit 6], KVM allows guest software to be able
>> > to set CR4.LASS.
>> > By design CR4.LASS can be manipulated by nested guest as
>> > well.
>> This is inaccurate. The change in nested_vmx_cr_fixed1_bits_update() is
>> to allow L1 guests to set CR4.LASS in VMX operation.
>
>Essentially it allows nested guest to set CR4.LASS. L1 guest uses cr4_fixed1
>to check
>cr4 value requested to set by nested guest valid or not. Nested guest will
>get #GP
>fault if it's not allowed.

The change to CR4_FIXED1 has more impacts. Without CR4_FIXED1 change,
guest shouldn't enable LASS in VMX operation; it means:

1. before VMXON, LASS should be disabled
2. in VMX operation, LASS cannot be enabled

What you said (i.e., L1 guest allows L2 to enable LASS) belongs in #2.
But #1 isn't covered. That's why I said "inaccurate".

>
>> I would say:
>> 
>> Set the CR4.LASS bit in the emulated IA32_VMX_CR4_FIXED1 MSR for guests
>> to allow guests to enable LASS in nested VMX operation.
>