Re: [PATCH v5 12/21] kvm: x86: Intercept #NM for saving IA32_XFD_ERR

[Sean Christopherson <seanjc@google.com>]

On Wed, Jan 05, 2022, Yang Zhong wrote:
> @@ -6399,6 +6424,26 @@ static void handle_interrupt_nmi_irqoff(struct kvm_vcpu *vcpu,
>  	kvm_after_interrupt(vcpu);
>  }
>  
> +static void handle_nm_fault_irqoff(struct kvm_vcpu *vcpu)
> +{
> +	/*
> +	 * Save xfd_err to guest_fpu before interrupt is enabled, so the
> +	 * MSR value is not clobbered by the host activity before the guest
> +	 * has chance to consume it.
> +	 *
> +	 * We should not blindly read xfd_err here, since this exception

Nit, avoid "we", and explain what KVM does (or doesn't) do, not what KVM "should"
do, e.g. just say

	 * Do not blindly read ...

> +	 * might be caused by L1 interception on a platform which doesn't
> +	 * support xfd at all.
> +	 *
> +	 * Do it conditionally upon guest_fpu::xfd. xfd_err matters
> +	 * only when xfd contains a non-zero value.
> +	 *
> +	 * Queuing exception is done in vmx_handle_exit. See comment there.

Another nit, it's worth explaining why XFD_ERR needs to be read here regardless
of is_guest_mode().  E.g.

	 * Injecting the #NM back into the guest is handled in the standard path
	 * as an #NM in L2 may be reflected into L1 as a VM-Exit.  Read XFD_ERR
	 * even if the #NM is from L2, as L1 may have exposed XFD to L2.

Side topic, in a follow up series/patch, it's probably worth adding support in
nested_vmx_prepare_msr_bitmap() to allow passthrough of the MSRs to L2.

> +	 */
> +	if (vcpu->arch.guest_fpu.fpstate->xfd)
> +		rdmsrl(MSR_IA32_XFD_ERR, vcpu->arch.guest_fpu.xfd_err);
> +}
> +
>  static void handle_exception_nmi_irqoff(struct vcpu_vmx *vmx)
>  {
>  	const unsigned long nmi_entry = (unsigned long)asm_exc_nmi_noist;
> @@ -6407,6 +6452,9 @@ static void handle_exception_nmi_irqoff(struct vcpu_vmx *vmx)
>  	/* if exit due to PF check for async PF */
>  	if (is_page_fault(intr_info))
>  		vmx->vcpu.arch.apf.host_apf_flags = kvm_read_and_reset_apf_flags();
> +	/* if exit due to NM, handle before interrupts are enabled */

Nit, drop this comment, it's slightly misleading since the #NM isn't fully handled
here.  The comment in handle_nm_fault_irqoff() is more than sufficient.

> +	else if (is_nm_fault(intr_info))
> +		handle_nm_fault_irqoff(&vmx->vcpu);
>  	/* Handle machine checks before interrupts are enabled */
>  	else if (is_machine_check(intr_info))
>  		kvm_machine_check();
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 21ce65220e38..2c988f8ca616 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -9953,6 +9953,9 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  	if (test_thread_flag(TIF_NEED_FPU_LOAD))
>  		switch_fpu_return();
>  
> +	if (vcpu->arch.guest_fpu.xfd_err)
> +		wrmsrl(MSR_IA32_XFD_ERR, vcpu->arch.guest_fpu.xfd_err);
> +
>  	if (unlikely(vcpu->arch.switch_db_regs)) {
>  		set_debugreg(0, 7);
>  		set_debugreg(vcpu->arch.eff_db[0], 0);
> @@ -10016,6 +10019,9 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  
>  	static_call(kvm_x86_handle_exit_irqoff)(vcpu);
>  
> +	if (vcpu->arch.guest_fpu.xfd_err)
> +		wrmsrl(MSR_IA32_XFD_ERR, 0);
> +
>  	/*
>  	 * Consume any pending interrupts, including the possible source of
>  	 * VM-Exit on SVM and any ticks that occur between VM-Exit and now.