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

[Zeng Guang]

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.

> 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.