linux-kernel - Re: [PATCH v6 2/3] KVM: selftests: Add a test to get/set triple fault event

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Date:   Wed, 18 May 2022 19:20:32 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Chenyi Qiang <chenyi.qiang@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Xiaoyao Li <xiaoyao.li@...el.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 2/3] KVM: selftests: Add a test to get/set triple
 fault event

On Thu, Apr 21, 2022, Chenyi Qiang wrote:
> +#ifndef __x86_64__
> +# error This test is 64-bit only

No need, all of KVM selftests are 64-bit only.

> +#endif
> +
> +#define VCPU_ID			0
> +#define ARBITRARY_IO_PORT	0x2000
> +
> +/* The virtual machine object. */
> +static struct kvm_vm *vm;
> +
> +static void l2_guest_code(void)
> +{
> +	/*
> +	 * Generate an exit to L0 userspace, i.e. main(), via I/O to an
> +	 * arbitrary port.
> +	 */

I think we can test a "real" triple fault without too much effort by abusing
vcpu->run->request_interrupt_window.  E.g. map the run struct into L2, clear
PIN_BASED_EXT_INTR_MASK in vmcs12, and then in l2_guest_code() do:

	asm volatile("cli");

	run->request_interrupt_window = true;

	asm volatile("sti; ud2");

	asm volatile("inb %%dx, %%al"                                           
		     : : [port] "d" (ARBITRARY_IO_PORT) : "rax"); 	

The STI shadow will block the IRQ-window VM-Exit until after the ud2, i.e. until
after the triple fault is pending.

And then main() can

  1. verify it got KVM_EXIT_IRQ_WINDOW_OPEN with a pending triple fault
  2. clear the triple fault and re-enter L1+l2
  3. continue with the existing code, e.g. verify it got KVM_EXIT_IO, stuff a
     pending triple fault, etc...

That said, typing that out makes me think we should technically handle/prevent this
in KVM since triple fault / shutdown is kinda sorta just a special exception.

Heh, and a potentially bad/crazy idea would be to use a reserved/magic vector in
kvm_vcpu_events.exception to save/restore triple fault, e.g. we could steal
NMI_VECTOR.

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index f2d0ee9296b9..d58c0cfd3cd3 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4743,7 +4743,8 @@ static int kvm_vcpu_ready_for_interrupt_injection(struct kvm_vcpu *vcpu)
        return (kvm_arch_interrupt_allowed(vcpu) &&
                kvm_cpu_accept_dm_intr(vcpu) &&
                !kvm_event_needs_reinjection(vcpu) &&
-               !vcpu->arch.exception.pending);
+               !vcpu->arch.exception.pending &&
+               !kvm_test_request(KVM_REQ_TRIPLE_FAULT, vcpu));
 }

 static int kvm_vcpu_ioctl_interrupt(struct kvm_vcpu *vcpu,