Re: [PATCH v1 5/5] selftests: KVM: SVM: Add Idle HLT intercept test

[Sean Christopherson <seanjc@google.com>]

On Thu, Mar 07, 2024, Sean Christopherson wrote:
> On Thu, Mar 07, 2024, Manali Shukla wrote: From: Manali Shukla <Manali.Shukla@amd.com>
> > +	xapic_enable();
> > +
> > +	icr_val = (APIC_DEST_SELF | APIC_INT_ASSERT | VINTR_VECTOR);
> > +
> > +	for (i = 0; i < NUM_ITERATIONS; i++) {
> > +		xapic_write_reg(APIC_ICR, icr_val);
> > +		asm volatile("sti; hlt; cli");
> 
> Please add safe_halt() and cli() helpers in processor.h.  And then do:

Doh, saw something shiny and forgot to finish my though.  For safe_halt(), copy
the thing verbatim from KVM-Unit-Tests, as not everyone is familiar with the
sti=>hlt trick.

/*
 * Execute HLT in an STI interrupt shadow to ensure that a pending IRQ that's
 * intended to be a wake event arrives *after* HLT is executed.  Modern CPUs,
 * except for a few oddballs that KVM is unlikely to run on, block IRQs for one
 * instruction after STI, *if* RFLAGS.IF=0 before STI.  Note, Intel CPUs may
 * block other events beyond regular IRQs, e.g. may block NMIs and SMIs too.
 */
static inline void safe_halt(void)
{
	asm volatile("sti; hlt");
}