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

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Message-ID: <Z4WmKTRQ8e-amGus@google.com>
Date: Mon, 13 Jan 2025 15:47:53 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Adrian Hunter <adrian.hunter@...el.com>
Cc: Chao Gao <chao.gao@...el.com>, pbonzini@...hat.com, kvm@...r.kernel.org, 
	dave.hansen@...ux.intel.com, rick.p.edgecombe@...el.com, kai.huang@...el.com, 
	reinette.chatre@...el.com, xiaoyao.li@...el.com, 
	tony.lindgren@...ux.intel.com, binbin.wu@...ux.intel.com, dmatlack@...gle.com, 
	isaku.yamahata@...el.com, nik.borisov@...e.com, linux-kernel@...r.kernel.org, 
	x86@...nel.org, yan.y.zhao@...el.com, weijiang.yang@...el.com
Subject: Re: [PATCH 4/7] KVM: TDX: restore host xsave state when exit from the
 guest TD

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.