Re: [RFC PATCH kernel 2/2] KVM: SEV: PreventHostIBS enablement for SEV-ES and SNP guest

[Sean Christopherson <seanjc@google.com>]

On Mon, Feb 06, 2023, Manali Shukla wrote:
> Currently, the hypervisor is able to inspect instruction based samples
> from a guest and gather execution information. SEV-ES and SNP guests
> can disallow the use of instruction based sampling by hypervisor by
> enabling the PreventHostIBS feature for the guest.  (More information
> in Section 15.36.17 APM Volume 2)
> 
> The MSR_AMD64_IBSFETCHCTL[IbsFetchEn] and MSR_AMD64_IBSOPCTL[IbsOpEn]
> bits need to be disabled before VMRUN is called when PreventHostIBS
> feature is enabled. If either of these bits are not 0, VMRUN will fail
> with VMEXIT_INVALID error code.
> 
> Because of an IBS race condition when disabling IBS, KVM needs to
> indicate when it is in a PreventHostIBS window. Activate the window
> based on whether IBS is currently active or inactive.
> 
> Signed-off-by: Manali Shukla <manali.shukla@amd.com>
> ---
>  arch/x86/include/asm/cpufeatures.h |  1 +
>  arch/x86/kvm/svm/sev.c             | 10 ++++++++
>  arch/x86/kvm/svm/svm.c             | 39 ++++++++++++++++++++++++++++--
>  arch/x86/kvm/svm/svm.h             |  1 +
>  4 files changed, 49 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> index 61012476d66e..1812e74f846a 100644
> --- a/arch/x86/include/asm/cpufeatures.h
> +++ b/arch/x86/include/asm/cpufeatures.h
> @@ -425,6 +425,7 @@
>  #define X86_FEATURE_SEV_ES		(19*32+ 3) /* AMD Secure Encrypted Virtualization - Encrypted State */
>  #define X86_FEATURE_V_TSC_AUX		(19*32+ 9) /* "" Virtual TSC_AUX */
>  #define X86_FEATURE_SME_COHERENT	(19*32+10) /* "" AMD hardware-enforced cache coherency */
> +#define X86_FEATURE_PREVENT_HOST_IBS	(19*32+15) /* "" AMD prevent host ibs */
>  
>  /*
>   * BUG word(s)
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 86d6897f4806..b348b8931721 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -569,6 +569,12 @@ static int sev_es_sync_vmsa(struct vcpu_svm *svm)
>  	if (svm->vcpu.guest_debug || (svm->vmcb->save.dr7 & ~DR7_FIXED_1))
>  		return -EINVAL;
>  
> +	if (sev_es_guest(svm->vcpu.kvm) &&
> +	    guest_cpuid_has(&svm->vcpu, X86_FEATURE_PREVENT_HOST_IBS)) {
> +		save->sev_features |= BIT(6);
> +		svm->prevent_hostibs_enabled = true;
> +	}
> +
>  	/*
>  	 * SEV-ES will use a VMSA that is pointed to by the VMCB, not
>  	 * the traditional VMSA that is part of the VMCB. Copy the
> @@ -2158,6 +2164,10 @@ void __init sev_set_cpu_caps(void)
>  		kvm_cpu_cap_clear(X86_FEATURE_SEV);
>  	if (!sev_es_enabled)
>  		kvm_cpu_cap_clear(X86_FEATURE_SEV_ES);
> +
> +	/* Enable PreventhostIBS feature for SEV-ES and higher guests */
> +	if (sev_es_enabled)
> +		kvm_cpu_cap_set(X86_FEATURE_PREVENT_HOST_IBS);

Uh, you can't just force a cap, there needs to be actual hardware support.  Just
copy what was done for X86_FEATURE_SEV_ES.


>  }
>  
>  void __init sev_hardware_setup(void)
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 9a194aa1a75a..47c1e0fff23e 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -3914,10 +3914,39 @@ static noinstr void svm_vcpu_enter_exit(struct kvm_vcpu *vcpu, bool spec_ctrl_in
>  
>  	guest_state_enter_irqoff();
>  
> -	if (sev_es_guest(vcpu->kvm))
> +	if (sev_es_guest(vcpu->kvm)) {
> +		bool ibs_fetch_active, ibs_op_active;
> +		u64 ibs_fetch_ctl, ibs_op_ctl;
> +
> +		if (svm->prevent_hostibs_enabled) {
> +			/*
> +			 * With PreventHostIBS enabled, IBS profiling cannot
> +			 * be active when VMRUN is executed. Disable IBS before
> +			 * executing VMRUN and, because of a race condition,
> +			 * enable the PreventHostIBS window if IBS profiling was
> +			 * active.

And the race can't be fixed because...?

> +			 */
> +			ibs_fetch_active =
> +				amd_disable_ibs_fetch(&ibs_fetch_ctl);
> +			ibs_op_active =
> +				amd_disable_ibs_op(&ibs_op_ctl);
> +
> +			amd_prevent_hostibs_window(ibs_fetch_active ||
> +						   ibs_op_active);
> +		}
> +
>  		__svm_sev_es_vcpu_run(svm, spec_ctrl_intercepted);
> -	else
> +
> +		if (svm->prevent_hostibs_enabled) {
> +			if (ibs_fetch_active)
> +				amd_restore_ibs_fetch(ibs_fetch_ctl);
> +
> +			if (ibs_op_active)
> +				amd_restore_ibs_op(ibs_op_ctl);

IIUC, this adds up to 2 RDMSRs and 4 WRMSRs to the VMRUN path.  Blech.  There's
gotta be a better way to implement this.  Like PeterZ said, this is basically
exclude_guest.