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

From: Sean Christopherson <sean.j.christopherson@intel.com>
To: Xiaoyao Li <xiaoyao.li@intel.com>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	Paolo Bonzini <pbonzini@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>
Subject: Re: [PATCH 1/2] kvm: vmx: Use basic exit reason to check if it's the specific VM EXIT
Date: Wed, 26 Feb 2020 15:59:24 -0800	[thread overview]
Message-ID: <20200226235924.GW9940@linux.intel.com> (raw)
In-Reply-To: <bb2d36b4-a077-691e-d59e-f65bf534d1ff@intel.com>

On Tue, Feb 25, 2020 at 02:41:20PM +0800, Xiaoyao Li wrote:
> On 2/25/2020 2:13 PM, Sean Christopherson wrote:
> >On Tue, Feb 25, 2020 at 08:13:15AM +0800, Xiaoyao Li wrote:
> >>On 2/25/2020 12:17 AM, Sean Christopherson wrote:
> >>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.
> 
> Well, for this reason we can keep exit_reason for 16-bit usage, and define
> full/extended_exit_reason for 32-bit cases. This makes less code churn.
> 
> But after we choose to use exit_reason and full/extended_exit_reason, what
> if someday new modifier flags are added and we want to enable some modifier
> flags for nested case?
> I guess we need to change existing exit_reason to full/extended_exit_reason
> in nested.c/nested.h to keep the naming rule consistent.

Ah, good point.  But, that's just another bug in my psuedo patch :-)
It's literally one call site that needs to be updated.  E.g.

	if (is_guest_mode(vcpu) && nested_vmx_exit_reflected(vcpu, exit_reason))
		return nested_vmx_reflect_vmexit(vcpu, full_exit_reason);

Everywhere else KVM calls nested_vmx_reflect_vmexit() is (currently) done
with a hardcoded value (except handle_vmfunc(), but I actually want to
change that one).