Re: [PATCH 4/7] KVM: TDX: restore host xsave state when exit from the guest TD

[Sean Christopherson <seanjc@google.com>]

On Mon, Jan 13, 2025, Adrian Hunter wrote:
> On 9/01/25 21:11, Sean Christopherson wrote:
> > My vote would to KVM_BUG_ON() before entering the guest.
> 
> I notice if KVM_BUG_ON() is called with interrupts disabled
> smp_call_function_many_cond() generates a warning:
> 
> WARNING: CPU: 223 PID: 4213 at kernel/smp.c:807 smp_call_function_many_cond+0x421/0x560
> 
> static void smp_call_function_many_cond(const struct cpumask *mask,
> 					smp_call_func_t func, void *info,
> 					unsigned int scf_flags,
> 					smp_cond_func_t cond_func)
> {
> 	int cpu, last_cpu, this_cpu = smp_processor_id();
> 	struct call_function_data *cfd;
> 	bool wait = scf_flags & SCF_WAIT;
> 	int nr_cpus = 0;
> 	bool run_remote = false;
> 	bool run_local = false;
> 
> 	lockdep_assert_preemption_disabled();
> 
> 	/*
> 	 * Can deadlock when called with interrupts disabled.
> 	 * We allow cpu's that are not yet online though, as no one else can
> 	 * send smp call function interrupt to this cpu and as such deadlocks
> 	 * can't happen.
> 	 */
> 	if (cpu_online(this_cpu) && !oops_in_progress &&
> 	    !early_boot_irqs_disabled)
> 		lockdep_assert_irqs_enabled();			<------------- here
> 
> Do we need to care about that?

Ugh, yes.  E.g. the deadlock mentioned in the comment would occur if two vCPUs
hit the KVM_BUG_ON() at the same time (they'd both wait for the other to respond
to *their* IPI).

Since the damage is limited to the current vCPU, i.e. letting userspace run other
vCPUs is unlikely to put KVM in harm's way, a not-awful alternative would be to
WARN_ON_ONCE() and return KVM_EXIT_INTERNAL_ERROR.