Re: [PATCH v3 07/11] KVM: VMX: Handle NMI Source report in VM exit

[Jacob Pan <jacob.jun.pan@linux.intel.com>]

On Mon, 1 Jul 2024 10:03:58 -0700, Xin Li <xin@zytor.com> wrote:

> On 7/1/2024 8:45 AM, Jacob Pan wrote:
> > 
> > On Fri, 28 Jun 2024 21:07:04 -0700, Xin Li <xin@zytor.com> wrote:
> >   
> >> On 6/28/2024 1:18 PM, Jacob Pan wrote:  
> >>> From: Zeng Guang <guang.zeng@intel.com>
> >>>
> >>> If the "NMI exiting" VM-execution control is 1, the value of the
> >>> 16-bit NMI source vector is saved in the exit-qualification field in
> >>> the VMCS when VM exits occur on CPUs that support NMI source.
> >>>
> >>> KVM that is aware of NMI-source reporting will push the bitmask of NMI
> >>> source vectors as the exceptoin event data field on the stack for then
> >>> entry of FRED exception. Subsequently, the host NMI exception handler
> >>> is invoked which will process NMI source information in the event
> >>> data. This operation is independent of vCPU FRED enabling status.
> >>>
> >>> Signed-off-by: Zeng Guang <guang.zeng@intel.com>
> >>> Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
> >>> ---
> >>>    arch/x86/entry/entry_64_fred.S |  2 +-
> >>>    arch/x86/kvm/vmx/vmx.c         | 11 ++++++++---
> >>>    2 files changed, 9 insertions(+), 4 deletions(-)
> >>>
> >>> diff --git a/arch/x86/entry/entry_64_fred.S
> >>> b/arch/x86/entry/entry_64_fred.S index a02bc6f3d2e6..0d934a3fcaf8
> >>> 100644 --- a/arch/x86/entry/entry_64_fred.S
> >>> +++ b/arch/x86/entry/entry_64_fred.S
> >>> @@ -92,7 +92,7 @@ SYM_FUNC_START(asm_fred_entry_from_kvm)
> >>>    	 * +--------+-----------------+
> >>>    	 */
> >>>    	push $0				/* Reserved, must
> >>> be 0 */
> >>> -	push $0				/* Event data, 0 for
> >>> IRQ/NMI */
> >>> +	push %rsi			/* Event data for IRQ/NMI */
> >>>    	push %rdi			/* fred_ss handed in by
> >>> the caller */ push %rbp
> >>>    	pushf  
> >>
> >> This belongs to the previous patch, it is part of the host changes.  
> > right, will do.
> >   
> >>  
> >>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> >>> index 4e7b36081b76..6719c598fa5f 100644
> >>> --- a/arch/x86/kvm/vmx/vmx.c
> >>> +++ b/arch/x86/kvm/vmx/vmx.c
> >>> @@ -7331,10 +7331,15 @@ static noinstr void vmx_vcpu_enter_exit(struct
> >>> kvm_vcpu *vcpu, if ((u16)vmx->exit_reason.basic ==
> >>> EXIT_REASON_EXCEPTION_NMI && is_nmi(vmx_get_intr_info(vcpu))) {
> >>>    		kvm_before_interrupt(vcpu, KVM_HANDLING_NMI);
> >>> -		if (cpu_feature_enabled(X86_FEATURE_FRED))
> >>> -			fred_entry_from_kvm(EVENT_TYPE_NMI,
> >>> NMI_VECTOR, 0);
> >>> -		else
> >>> +		if (cpu_feature_enabled(X86_FEATURE_FRED)) {
> >>> +			unsigned long edata = 0;
> >>> +
> >>> +			if
> >>> (cpu_feature_enabled(X86_FEATURE_NMI_SOURCE))
> >>> +				edata = vmx_get_exit_qual(vcpu);
> >>> +			fred_entry_from_kvm(EVENT_TYPE_NMI,
> >>> NMI_VECTOR, edata);  
> >>
> >> Simply do fred_entry_from_kvm(EVENT_TYPE_NMI, NMI_VECTOR,
> >> vmx_get_exit_qual(vcpu))?  
> > I don't have strong preference but having a local variable improves
> > readability IMHO.  
> 
> My point was, do we actually need this check:
>      (cpu_feature_enabled(X86_FEATURE_NMI_SOURCE)?
I also pondered about the value of this CPUID bit if it is consistently
linked with the FRED bit. But since the architecture provided this
additional enumeration, as noted in Chapter 9 of the FRED specification,
which states: 

"Processors that support FRED *may* also support a related feature called
NMI-source reporting".

The use of "may" suggests that there are scenarios where FRED might exist
without NMI-source reporting. To ensure future compatibility, I believe
it is still valid to maintain this check.


Thanks,

Jacob