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

[Sean Christopherson]

On Tue, Feb 25, 2020 at 08:13:15AM +0800, Xiaoyao Li wrote:
> 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@xxxxxxxxx> writes:
> >>
> >>>On 2/24/2020 6:16 PM, Vitaly Kuznetsov wrote:
> >>>>Xiaoyao Li <xiaoyao.li@xxxxxxxxx> writes:
> >>>>
> >>>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?

It's actually the opposite, almost all of the VMX code assumes exit_reason
holds only the basic exit reason, i.e. a 16-bit value.  For example, SGX
adds a modifier flag to denote a VM-Exit was from enclave mode, and that
bit needs to be stripped from exit_reason, otherwise all the checks like
"if (exit_reason == blah_blah_blah)" fail.  

Making exit_reason a 16-bit alias of the full/extended exit_reason neatly
sidesteps that issue.  And it is an issue that has caused actual problems
in the past, e.g. see commit beb8d93b3e42 ("KVM: VMX: Fix handling of #MC
that occurs during VM-Entry").  Coincidentally, that commit also removes a
local "u16 basic_exit_reason" :-).

Except for one mistake, the pseudo-patch below is the entirety of required
changes.  Most (all?) of the functions that take "u32 exit_reason" can (and
should) continue to take a u32.

As for the name, I strongly prefer keeping the exit_reason name for the
basic exit reason.  The vast majority of VM-Exits do not have modifiers
set, i.e. "basic exit reason" == vmcs.EXIT_REASON for nearly all normal
usage.  This holds true in every form of communication, e.g. when discussing
VM-Exit reasons, it's never qualified with "basic", it's simply the exit
reason.  IMO the code is better off following the colloquial usage of "exit
reason".  A simple comment above the union would suffice to clear up any
confusion with respect to the SDM.

> 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) {

If we do go the union route, this snippet of code is insufficient, the
full/extended exit reason needs to be snapshotted early for use in the
tracepoint and in fail_entry.hardware_entry_failure_reason.

> >                 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
> >>
>