linux-kernel - Re: [PATCH 1/2] kvm: vmx: Use basic exit reason to check if it's the specific VM EXIT

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Date:   Tue, 25 Feb 2020 08:13:15 +0800
From:   Xiaoyao Li <xiaoyao.li@...el.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH 1/2] kvm: vmx: Use basic exit reason to check if it's the
 specific VM EXIT

On 2/25/2020 12:17 AM, Sean Christopherson wrote:
> On Mon, Feb 24, 2020 at 02:04:46PM +0100, Vitaly Kuznetsov wrote:
>> Xiaoyao Li <xiaoyao.li@...el.com> writes:
>>
>>> On 2/24/2020 6:16 PM, Vitaly Kuznetsov wrote:
>>>> Xiaoyao Li <xiaoyao.li@...el.com> writes:
>>>>
>>
>> ...
>>
>>>>>    		rip = kvm_rip_read(vcpu);
>>>>>    		rip += vmcs_read32(VM_EXIT_INSTRUCTION_LEN);
>>>>>    		kvm_rip_write(vcpu, rip);
>>>>> @@ -5797,6 +5797,7 @@ static int vmx_handle_exit(struct kvm_vcpu *vcpu,
>>>>>    {
>>>>>    	struct vcpu_vmx *vmx = to_vmx(vcpu);
>>>>>    	u32 exit_reason = vmx->exit_reason;
>>>>> +	u16 basic_exit_reason = basic(exit_reason);
>>>>
>>>> I don't think renaming local variable is needed, let's just do
>>>>
>>>> 'u16 exit_reason = basic_exit_reason(vmx->exit_reason)' and keep the
>>>> rest of the code as-is.
>>>
>>> No, we can't do this.
>>>
>>> It's not just renaming local variable, the full 32-bit exit reason is
>>> used elsewhere in this function that needs the upper 16-bit.
>>>
>>> Here variable basic_exit_reason is added for the cases where only basic
>>> exit reason number is needed.
>>>
>>
>> Can we do the other way around, i.e. introduce 'extended_exit_reason'
>> and use it where all 32 bits are needed? I'm fine with the change, just
>> trying to minimize the (unneeded) code churn.
> 
> 100% agree.  Even better than adding a second field to vcpu_vmx would be
> to make it a union, though we'd probably want to call it something like
> full_exit_reason in that case.  That should give us compile-time checks on
> exit_reason, e.g. if we try to query one of the upper bits using a u16, e.g.

I have thought about union, but it seems

union {
	u16 exit_reason;
	u32 full_exit_reason;
}

is not a good name. Since there are many codes in vmx.c and nested.c 
assume that exit_reason stands for 32-bit EXIT REASON vmcs field as well 
as evmcs->vm_exit_reason and vmcs12->vm_exit_reason. Do we really want 
to also rename them to full_exit_reason?

Maybe we name it

union {
	u16 basic_exit_reason;
	u32 exit_reason;
}

as what SDM defines?

> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -5818,7 +5818,7 @@ static int vmx_handle_exit(struct kvm_vcpu *vcpu,
>          if (is_guest_mode(vcpu) && nested_vmx_exit_reflected(vcpu, exit_reason))
>                  return nested_vmx_reflect_vmexit(vcpu, exit_reason);
> 
> -       if (exit_reason & VMX_EXIT_REASONS_FAILED_VMENTRY) {
> +       if (vmx->full_exit_reason & VMX_EXIT_REASONS_FAILED_VMENTRY) {
>                  dump_vmcs();
>                  vcpu->run->exit_reason = KVM_EXIT_FAIL_ENTRY;
>                  vcpu->run->fail_entry.hardware_entry_failure_reason
> @@ -6620,11 +6620,12 @@ 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->full_exit_reason = vmx->fail ? 0xdead : vmcs_read32(VM_EXIT_REASON);
> +       if (vmx->exit_reason == EXIT_REASON_MCE_DURING_VMENTRY)
>                  kvm_machine_check();
> 
> -       if (vmx->fail || (vmx->exit_reason & VMX_EXIT_REASONS_FAILED_VMENTRY))
> +       if (vmx->fail ||
> +           (vmx->full_exit_reason & VMX_EXIT_REASONS_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 7f42cf3dcd70..60c09640ea59 100644
> --- a/arch/x86/kvm/vmx/vmx.h
> +++ b/arch/x86/kvm/vmx/vmx.h
> @@ -260,7 +260,10 @@ struct vcpu_vmx {
>          int vpid;
>          bool emulation_required;
> 
> -       u32 exit_reason;
> +       union {
> +               u16 exit_reason;
> +               u32 full_exit_reason;
> +       }
> 
>          /* Posted interrupt descriptor */
>          struct pi_desc pi_desc;
> 
> 
> 
> 
> 
>> -- 
>> Vitaly
>>