Re: [PATCH v2] KVM: x86: Always set kvm_run->if_flag

[Maxim Levitsky <mlevitsk@redhat.com>]

On Thu, 2021-12-09 at 07:52 -0800, Marc Orr wrote:
> The kvm_run struct's if_flag is a part of the userspace/kernel API. The
> SEV-ES patches failed to set this flag because it's no longer needed by
> QEMU (according to the comment in the source code). However, other
> hypervisors may make use of this flag. Therefore, set the flag for
> guests with encrypted registers (i.e., with guest_state_protected set).
> 
> Fixes: f1c6366e3043 ("KVM: SVM: Add required changes to support intercepts under SEV-ES")
> Signed-off-by: Marc Orr <marcorr@google.com>
> ---
> v1 -> v2
> - fix typos in commit message
> - fix `svm_get_if_flag()` to work for non-SEV
> - remove !! in return for both [svm|vmx]_get_if_flag()
> - refactor `svm_interrupt_blocked()` to use `svm_get_if_flag()` arch/x86/include/asm/kvm-x86-ops.h |  1 +
> 
>  arch/x86/include/asm/kvm_host.h    |  1 +
>  arch/x86/kvm/svm/svm.c             | 21 ++++++++++++---------
>  arch/x86/kvm/vmx/vmx.c             |  6 ++++++
>  arch/x86/kvm/x86.c                 |  9 +--------
>  5 files changed, 21 insertions(+), 17 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm-x86-ops.h b/arch/x86/include/asm/kvm-x86-ops.h
> index cefe1d81e2e8..9e50da3ed01a 100644
> --- a/arch/x86/include/asm/kvm-x86-ops.h
> +++ b/arch/x86/include/asm/kvm-x86-ops.h
> @@ -47,6 +47,7 @@ KVM_X86_OP(set_dr7)
>  KVM_X86_OP(cache_reg)
>  KVM_X86_OP(get_rflags)
>  KVM_X86_OP(set_rflags)
> +KVM_X86_OP(get_if_flag)
>  KVM_X86_OP(tlb_flush_all)
>  KVM_X86_OP(tlb_flush_current)
>  KVM_X86_OP_NULL(tlb_remote_flush)
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 860ed500580c..a7f868ff23e7 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1349,6 +1349,7 @@ struct kvm_x86_ops {
>  	void (*cache_reg)(struct kvm_vcpu *vcpu, enum kvm_reg reg);
>  	unsigned long (*get_rflags)(struct kvm_vcpu *vcpu);
>  	void (*set_rflags)(struct kvm_vcpu *vcpu, unsigned long rflags);
> +	bool (*get_if_flag)(struct kvm_vcpu *vcpu);
>  
>  	void (*tlb_flush_all)(struct kvm_vcpu *vcpu);
>  	void (*tlb_flush_current)(struct kvm_vcpu *vcpu);
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index d0f68d11ec70..5151efa424ac 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -1585,6 +1585,15 @@ static void svm_set_rflags(struct kvm_vcpu *vcpu, unsigned long rflags)
>  	to_svm(vcpu)->vmcb->save.rflags = rflags;
>  }
>  
> +static bool svm_get_if_flag(struct kvm_vcpu *vcpu)
> +{
> +	struct vmcb *vmcb = to_svm(vcpu)->vmcb;
> +
> +	return sev_es_guest(vcpu->kvm)
> +		? vmcb->control.int_state & SVM_GUEST_INTERRUPT_MASK
> +		: kvm_get_rflags(vcpu) & X86_EFLAGS_IF;
> +}
> +
>  static void svm_cache_reg(struct kvm_vcpu *vcpu, enum kvm_reg reg)
>  {
>  	switch (reg) {
> @@ -3568,14 +3577,7 @@ bool svm_interrupt_blocked(struct kvm_vcpu *vcpu)
>  	if (!gif_set(svm))
>  		return true;
>  
> -	if (sev_es_guest(vcpu->kvm)) {
> -		/*
> -		 * SEV-ES guests to not expose RFLAGS. Use the VMCB interrupt mask
> -		 * bit to determine the state of the IF flag.
> -		 */
> -		if (!(vmcb->control.int_state & SVM_GUEST_INTERRUPT_MASK))
> -			return true;
> -	} else if (is_guest_mode(vcpu)) {
> +	if (is_guest_mode(vcpu)) {
>  		/* As long as interrupts are being delivered...  */
>  		if ((svm->nested.ctl.int_ctl & V_INTR_MASKING_MASK)
>  		    ? !(svm->vmcb01.ptr->save.rflags & X86_EFLAGS_IF)
> @@ -3586,7 +3588,7 @@ bool svm_interrupt_blocked(struct kvm_vcpu *vcpu)
>  		if (nested_exit_on_intr(svm))
>  			return false;
>  	} else {
> -		if (!(kvm_get_rflags(vcpu) & X86_EFLAGS_IF))
> +		if (!svm_get_if_flag(vcpu))
>  			return true;
>  	}
>  
> @@ -4621,6 +4623,7 @@ static struct kvm_x86_ops svm_x86_ops __initdata = {
>  	.cache_reg = svm_cache_reg,
>  	.get_rflags = svm_get_rflags,
>  	.set_rflags = svm_set_rflags,
> +	.get_if_flag = svm_get_if_flag,
>  
>  	.tlb_flush_all = svm_flush_tlb,
>  	.tlb_flush_current = svm_flush_tlb,
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 9453743ce0c4..269de5bb98d7 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1363,6 +1363,11 @@ void vmx_set_rflags(struct kvm_vcpu *vcpu, unsigned long rflags)
>  		vmx->emulation_required = vmx_emulation_required(vcpu);
>  }
>  
> +static bool vmx_get_if_flag(struct kvm_vcpu *vcpu)
> +{
> +	return vmx_get_rflags(vcpu) & X86_EFLAGS_IF;
> +}
> +
>  u32 vmx_get_interrupt_shadow(struct kvm_vcpu *vcpu)
>  {
>  	u32 interruptibility = vmcs_read32(GUEST_INTERRUPTIBILITY_INFO);
> @@ -7575,6 +7580,7 @@ static struct kvm_x86_ops vmx_x86_ops __initdata = {
>  	.cache_reg = vmx_cache_reg,
>  	.get_rflags = vmx_get_rflags,
>  	.set_rflags = vmx_set_rflags,
> +	.get_if_flag = vmx_get_if_flag,
>  
>  	.tlb_flush_all = vmx_flush_tlb_all,
>  	.tlb_flush_current = vmx_flush_tlb_current,
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index e0aa4dd53c7f..45e836db5bcd 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -8995,14 +8995,7 @@ static void post_kvm_run_save(struct kvm_vcpu *vcpu)
>  {
>  	struct kvm_run *kvm_run = vcpu->run;
>  
> -	/*
> -	 * if_flag is obsolete and useless, so do not bother
> -	 * setting it for SEV-ES guests.  Userspace can just
> -	 * use kvm_run->ready_for_interrupt_injection.
> -	 */
> -	kvm_run->if_flag = !vcpu->arch.guest_state_protected
> -		&& (kvm_get_rflags(vcpu) & X86_EFLAGS_IF) != 0;
> -
> +	kvm_run->if_flag = static_call(kvm_x86_get_if_flag)(vcpu);
>  	kvm_run->cr8 = kvm_get_cr8(vcpu);
>  	kvm_run->apic_base = kvm_get_apic_base(vcpu);
>  

Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>

Best regards,
	Maxim Levitsky