Re: [PATCH 10/10] KVM: VMX: Convert vcpu_vmx.exit_reason to a union

From: Vitaly Kuznetsov
Date: Fri Mar 13 2020 - 10:18:23 EST

Sean Christopherson <sean.j.christopherson@xxxxxxxxx> writes:

> Convert vcpu_vmx.exit_reason from a u32 to a union (of size u32). The
> full VM_EXIT_REASON field is comprised of a 16-bit basic exit reason in
> bits 15:0, and single-bit modifiers in bits 31:16.
>
> Historically, KVM has only had to worry about handling the "failed
> VM-Entry" modifier, which could only be set in very specific flows and
> required dedicated handling. I.e. manually stripping the FAILED_VMENTRY
> bit was a somewhat viable approach. But even with only a single bit to
> worry about, KVM has had several bugs related to comparing a basic exit
> reason against the full exit reason stored in vcpu_vmx.
>
> Upcoming Intel features, e.g. SGX, will add new modifier bits that can
> be set on more or less any VM-Exit, as opposed to the significantly more
> restricted FAILED_VMENTRY, i.e. correctly handling everything in one-off
> flows isn't scalable. Tracking exit reason in a union forces code to
> explicitly choose between consuming the full exit reason and the basic
> exit reason, and is a convenient way to document and access the
> modifiers.
>
> No functional change intended.
>
> Cc: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> ---
> arch/x86/kvm/vmx/nested.c | 11 ++++++++---
> arch/x86/kvm/vmx/nested.h | 2 +-
> arch/x86/kvm/vmx/vmx.c | 24 ++++++++++++------------
> arch/x86/kvm/vmx/vmx.h | 25 ++++++++++++++++++++++++-
> 4 files changed, 45 insertions(+), 17 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index c775feca3eb0..0c7cea35dd33 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -5307,7 +5307,12 @@ static int handle_vmfunc(struct kvm_vcpu *vcpu)
> return kvm_skip_emulated_instruction(vcpu);
>
> fail:
> - nested_vmx_vmexit(vcpu, vmx->exit_reason,
> + /*
> + * This is effectively a reflected VM-Exit, as opposed to a synthesized
> + * nested VM-Exit. Pass the original exit reason, i.e. don't hardcode
> + * EXIT_REASON_VMFUNC as the exit reason.
> + */
> + nested_vmx_vmexit(vcpu, vmx->exit_reason.full,
> vmcs_read32(VM_EXIT_INTR_INFO),
> vmcs_readl(EXIT_QUALIFICATION));
> return 1;
> @@ -5549,14 +5554,14 @@ bool nested_vmx_exit_reflected(struct kvm_vcpu *vcpu)
> */
> nested_mark_vmcs12_pages_dirty(vcpu);
>
> - trace_kvm_nested_vmexit(kvm_rip_read(vcpu), vmx->exit_reason,
> + trace_kvm_nested_vmexit(kvm_rip_read(vcpu), vmx->exit_reason.full,
> vmcs_readl(EXIT_QUALIFICATION),
> vmx->idt_vectoring_info,
> intr_info,
> vmcs_read32(VM_EXIT_INTR_ERROR_CODE),
> KVM_ISA_VMX);
>
> - exit_reason = vmx->exit_reason;
> + exit_reason = vmx->exit_reason.basic;
>
> switch (exit_reason) {
> case EXIT_REASON_EXCEPTION_NMI:
> diff --git a/arch/x86/kvm/vmx/nested.h b/arch/x86/kvm/vmx/nested.h
> index 04584bcbcc8d..07ce09f88977 100644
> --- a/arch/x86/kvm/vmx/nested.h
> +++ b/arch/x86/kvm/vmx/nested.h
> @@ -98,7 +98,7 @@ static inline bool nested_vmx_reflect_vmexit(struct kvm_vcpu *vcpu)
> vmcs_read32(VM_EXIT_INTR_ERROR_CODE);
> }
>
> - nested_vmx_vmexit(vcpu, to_vmx(vcpu)->exit_reason, exit_intr_info,
> + nested_vmx_vmexit(vcpu, to_vmx(vcpu)->exit_reason.full, exit_intr_info,
> vmcs_readl(EXIT_QUALIFICATION));
> return true;
> }
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 910a7cadeaf7..521b99f63608 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1588,7 +1588,7 @@ static int skip_emulated_instruction(struct kvm_vcpu *vcpu)
> * i.e. we end up advancing IP with some random value.
> */
> if (!static_cpu_has(X86_FEATURE_HYPERVISOR) ||
> - to_vmx(vcpu)->exit_reason != EXIT_REASON_EPT_MISCONFIG) {
> + to_vmx(vcpu)->exit_reason.basic != EXIT_REASON_EPT_MISCONFIG) {
> rip = kvm_rip_read(vcpu);
> rip += vmcs_read32(VM_EXIT_INSTRUCTION_LEN);
> kvm_rip_write(vcpu, rip);
> @@ -5847,7 +5847,7 @@ static int vmx_handle_exit(struct kvm_vcpu *vcpu,
> u32 vectoring_info = vmx->idt_vectoring_info;
> u16 exit_reason;
>
> - trace_kvm_exit(vmx->exit_reason, vcpu, KVM_ISA_VMX);
> + trace_kvm_exit(vmx->exit_reason.full, vcpu, KVM_ISA_VMX);
>
> /*
> * Flush logged GPAs PML buffer, this will make dirty_bitmap more
> @@ -5866,11 +5866,11 @@ static int vmx_handle_exit(struct kvm_vcpu *vcpu,
> if (is_guest_mode(vcpu) && nested_vmx_reflect_vmexit(vcpu))
> return 1;
>
> - if (vmx->exit_reason & VMX_EXIT_REASONS_FAILED_VMENTRY) {
> + if (vmx->exit_reason.failed_vmentry) {
> dump_vmcs();
> vcpu->run->exit_reason = KVM_EXIT_FAIL_ENTRY;
> vcpu->run->fail_entry.hardware_entry_failure_reason
> - = vmx->exit_reason;
> + = vmx->exit_reason.full;
> return 0;
> }
>
> @@ -5882,7 +5882,7 @@ static int vmx_handle_exit(struct kvm_vcpu *vcpu,
> return 0;
> }
>
> - exit_reason = vmx->exit_reason;
> + exit_reason = vmx->exit_reason.basic;
>
> /*
> * Note:
> @@ -5900,7 +5900,7 @@ static int vmx_handle_exit(struct kvm_vcpu *vcpu,
> vcpu->run->internal.suberror = KVM_INTERNAL_ERROR_DELIVERY_EV;
> vcpu->run->internal.ndata = 3;
> vcpu->run->internal.data[0] = vectoring_info;
> - vcpu->run->internal.data[1] = vmx->exit_reason;
> + vcpu->run->internal.data[1] = vmx->exit_reason.full;
> vcpu->run->internal.data[2] = vcpu->arch.exit_qualification;
> if (exit_reason == EXIT_REASON_EPT_MISCONFIG) {
> vcpu->run->internal.ndata++;
> @@ -5960,13 +5960,13 @@ static int vmx_handle_exit(struct kvm_vcpu *vcpu,
>
> unexpected_vmexit:
> vcpu_unimpl(vcpu, "vmx: unexpected exit reason 0x%x\n",
> - vmx->exit_reason);
> + vmx->exit_reason.full);
> dump_vmcs();
> vcpu->run->exit_reason = KVM_EXIT_INTERNAL_ERROR;
> vcpu->run->internal.suberror =
> KVM_INTERNAL_ERROR_UNEXPECTED_EXIT_REASON;
> vcpu->run->internal.ndata = 1;
> - vcpu->run->internal.data[0] = vmx->exit_reason;
> + vcpu->run->internal.data[0] = vmx->exit_reason.full;
> return 0;
> }
>
> @@ -6290,7 +6290,7 @@ static void vmx_handle_exit_irqoff(struct kvm_vcpu *vcpu,
> enum exit_fastpath_completion *exit_fastpath)
> {
> struct vcpu_vmx *vmx = to_vmx(vcpu);
> - u16 exit_reason = vmx->exit_reason;
> + u16 exit_reason = vmx->exit_reason.basic;
>
> if (exit_reason == EXIT_REASON_EXTERNAL_INTERRUPT)
> handle_external_interrupt_irqoff(vcpu);
> @@ -6672,11 +6672,11 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
> vmx->nested.nested_run_pending = 0;
> vmx->idt_vectoring_info = 0;
>
> - vmx->exit_reason = vmx->fail ? 0xdead : vmcs_read32(VM_EXIT_REASON);
> - if ((u16)vmx->exit_reason == EXIT_REASON_MCE_DURING_VMENTRY)
> + vmx->exit_reason.full = vmx->fail ? 0xdead : vmcs_read32(VM_EXIT_REASON);
> + if (vmx->exit_reason.basic == EXIT_REASON_MCE_DURING_VMENTRY)
> kvm_machine_check();
>
> - if (vmx->fail || (vmx->exit_reason & VMX_EXIT_REASONS_FAILED_VMENTRY))
> + if (vmx->fail || vmx->exit_reason.failed_vmentry)
> return;
>
> vmx->loaded_vmcs->launched = 1;
> diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
> index e64da06c7009..2d9a005d11ab 100644
> --- a/arch/x86/kvm/vmx/vmx.h
> +++ b/arch/x86/kvm/vmx/vmx.h
> @@ -93,6 +93,29 @@ struct pt_desc {
> struct pt_ctx guest;
> };
>
> +union vmx_exit_reason {
> + struct {
> + u32 basic : 16;
> + u32 reserved16 : 1;
> + u32 reserved17 : 1;
> + u32 reserved18 : 1;
> + u32 reserved19 : 1;
> + u32 reserved20 : 1;
> + u32 reserved21 : 1;
> + u32 reserved22 : 1;
> + u32 reserved23 : 1;
> + u32 reserved24 : 1;
> + u32 reserved25 : 1;
> + u32 reserved26 : 1;
> + u32 enclave_mode : 1;
> + u32 smi_pending_mtf : 1;
> + u32 smi_from_vmx_root : 1;
> + u32 reserved30 : 1;
> + u32 failed_vmentry : 1;

Just wondering, is there any particular benefit in using 'u32' instead
of 'u16' here?

> + };
> + u32 full;
> +};
> +
> /*
> * The nested_vmx structure is part of vcpu_vmx, and holds information we need
> * for correct emulation of VMX (i.e., nested VMX) on this vcpu.
> @@ -263,7 +286,7 @@ struct vcpu_vmx {
> int vpid;
> bool emulation_required;
>
> - u32 exit_reason;
> + union vmx_exit_reason exit_reason;
>
> /* Posted interrupt descriptor */
> struct pi_desc pi_desc;

Reviewed-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>

--
Vitaly