Re: [RFC PATCH v2 04/11] KVM: VMX: Add IA32_SPEC_CTRL virtualization support

[Binbin Wu]

On 4/14/2023 2:25 PM, Chao Gao wrote:
> From: Zhang Chen <chen.zhang@xxxxxxxxx>
>
> Currently KVM disables interception of IA32_SPEC_CTRL after a non-0 is
> written to IA32_SPEC_CTRL by guest. Then, guest is allowed to write any
> value to hardware.
>
> "virtualize IA32_SPEC_CTRL" is a new tertiary vm-exec control. This
> feature allows KVM to specify that certain bits of the IA32_SPEC_CTRL
> MSR cannot be modified by guest software.
>
> Two VMCS fields are added:
>
>    IA32_SPEC_CTRL_MASK:   bits that guest software cannot modify
>    IA32_SPEC_CTRL_SHADOW: value that guest software expects to be in the
> 			 IA32_SPEC_CTRL MSR
>
> On rdmsr, the shadow value is returned. on wrmsr, EDX:EAX is written
> to the IA32_SPEC_CTRL_SHADOW and (cur_val & mask) | (EDX:EAX & ~mask)
> is written to the IA32_SPEC_CTRL MSR, where
>    * cur_val is the original value of IA32_SPEC_CTRL MSR
>    * mask is the value of IA32_SPEC_CTRL_MASK
>
> Add a mask e.g.,

e.g. or i.e. ?


> loaded_vmcs->spec_ctrl_mask to represent the bits guest
> shouldn't change. It is 0 for now and some bits will be added by
> following patches. Use per-vmcs cache to avoid unnecessary vmcs_write()
> on nested transition because the mask is expected to be rarely changed
> and the same for vmcs01 and vmcs02.
>
> To prevent guest from changing the bits in the mask, enable "virtualize
> IA32_SPEC_CTRL" if supported or emulate its behavior by intercepting
> the IA32_SPEC_CTRL msr. Emulating "virtualize IA32_SPEC_CTRL" behavior
> is mainly to give the same capability to KVM running on potential broken
> hardware or L1 guests.
>
> To avoid L2 evading the enforcement, enable "virtualize IA32_SPEC_CTRL"
> in vmcs02. Always update the guest (shadow) value of IA32_SPEC_CTRL MSR
> and the mask to preserve them across nested transitions. Note that the
> shadow value may be changed because L2 may access the IA32_SPEC_CTRL
> directly and the mask may be changed due to migration when L2 vCPUs are
> running.
>
> Co-developed-by: Chao Gao <chao.gao@xxxxxxxxx>
> Signed-off-by: Chao Gao <chao.gao@xxxxxxxxx>
> Signed-off-by: Zhang Chen <chen.zhang@xxxxxxxxx>
> Signed-off-by: Chao Gao <chao.gao@xxxxxxxxx>
> Tested-by: Jiaan Lu <jiaan.lu@xxxxxxxxx>
> ---
>   arch/x86/include/asm/vmx.h         |  5 ++++
>   arch/x86/include/asm/vmxfeatures.h |  2 ++
>   arch/x86/kvm/vmx/capabilities.h    |  5 ++++
>   arch/x86/kvm/vmx/nested.c          | 13 ++++++++++
>   arch/x86/kvm/vmx/vmcs.h            |  2 ++
>   arch/x86/kvm/vmx/vmx.c             | 34 ++++++++++++++++++++-----
>   arch/x86/kvm/vmx/vmx.h             | 40 +++++++++++++++++++++++++++++-
>   7 files changed, 94 insertions(+), 7 deletions(-)
[...]

> @@ -750,4 +766,26 @@ static inline bool guest_cpuid_has_evmcs(struct kvm_vcpu *vcpu)
>   	       to_vmx(vcpu)->nested.enlightened_vmcs_enabled;
>   }
>   
> +static inline u64 vmx_get_guest_spec_ctrl(struct vcpu_vmx *vmx)
> +{
> +	return vmx->guest_spec_ctrl;
> +}
> +
> +static inline void vmx_set_guest_spec_ctrl(struct vcpu_vmx *vmx, u64 val)
> +{
> +	vmx->guest_spec_ctrl = val;
> +
> +	/*
> +	 * For simplicity, always keep IA32_SPEC_CTRL_SHADOW up-to-date,
> +	 * regardless of the MSR intercept state.

It is better to use "IA32_SPEC_CTRL"  explicitly instead of "the MSR" to 
avoid misunderstand.


> +	 */
> +	if (cpu_has_spec_ctrl_virt())
> +		vmcs_write64(IA32_SPEC_CTRL_SHADOW, val);
> +
> +	/*
> +	 * Update the effective value of IA32_SPEC_CTRL to reflect changes to
> +	 * guest's IA32_SPEC_CTRL. Bits in the mask should always be set.
> +	 */
> +	vmx->spec_ctrl = val | vmx_get_spec_ctrl_mask(vmx);
> +}
>   #endif /* __KVM_X86_VMX_H */