Re: [PATCH v6 1/9] x86/fred, KVM: VMX: Pass event data to the FRED entry point from KVM

[Sean Christopherson <seanjc@google.com>]

On Wed, May 14, 2025, Sohil Mehta wrote:
> On 5/14/2025 7:15 AM, Sean Christopherson wrote:
> > Compile tested only...
> > 
> 
> No worries. I'll test it out. I am assuming you want this patch to go as
> part of this series.

Yes please.  I can also post it separately, but that seems unnecessary.

> > --
> > From: Sean Christopherson <seanjc@google.com>
> > Date: Wed, 14 May 2025 07:07:55 -0700
> > Subject: [PATCH] x86/fred: Provide separate IRQ vs. NMI wrappers for "entry"
> >  from KVM
> > 
> > Provide separate wrappers for forwarding IRQs vs NMIs from KVM in
> > anticipation of adding support for NMI source reporting, which will add
> > an NMI-only parameter, i.e. will further pollute the current API with a
> > param that is a hardcoded for one of the two call sites.
> > 
> > Opportunistically tag the non-FRED NMI wrapper __always_inline, as the
> > compiler could theoretically generate a function call and trigger and a
> > (completely benign) "leaving noinstr" warning.
> > 
> 
> If this is really a concern, wouldn't there be similar semantics in
> other places as well?

There are, e.g. the stubs in include/linux/context_tracking_state.h and many
other places.  It looks ridiculous, but the compiler will generate RET+CALL for
literal nops if the right sanitizers are enabled.  E.g. see commit
432727f1cb6e ("KVM: VMX: Always inline to_vmx() and to_kvm_vmx()").

> > @@ -70,14 +71,26 @@ __visible void fred_entry_from_user(struct pt_regs *regs);
> >  __visible void fred_entry_from_kernel(struct pt_regs *regs);
> >  __visible void __fred_entry_from_kvm(struct pt_regs *regs);
> >  
> > -/* Can be called from noinstr code, thus __always_inline */
> > -static __always_inline void fred_entry_from_kvm(unsigned int type, unsigned int vector)
> > +/* Must be called from noinstr code, thus __always_inline */
> > +static __always_inline void fred_nmi_from_kvm(void)
> >  {
> >  	struct fred_ss ss = {
> >  		.ss     =__KERNEL_DS,
> > -		.type   = type,
> > +		.type   = EVENT_TYPE_NMI,
> > +		.vector = NMI_VECTOR,
> > +		.nmi    = true,
> > +		.lm     = 1,
> > +	};
> > +
> > +	asm_fred_entry_from_kvm(ss);
> > +}
> > +
> 
> The original code uses spaces for alignment. Since we are modifying it,
> I am thinking of changing it to tabs.

Oof, yeah, definitely do that.