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

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Message-ID: <You1Eq6fp8F3YF5Z@google.com>
Date:   Mon, 23 May 2022 16:23:46 +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 Mon, May 23, 2022, Chenyi Qiang wrote:
> 
> 
> On 5/19/2022 3:20 AM, Sean Christopherson wrote:
> > 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;
> > 
> 
> Maybe, A GUEST_SYNC to main() to change the request_interrupt_window also
> works.

Hmm, yes, that should work too.  Feel free to punt on writing this sub-test.  As
mentioned below, KVM should treat a pending triple fault as a pending "exception",
i.e. userspace shouldn't see KVM_EXIT_IRQ_WINDOW_OPEN with a pending triple fault,
and so the test should actually assert that the triple fault occurs in L2.  But I
can roll that test into the KVM fix if you'd like.

> > 	asm volatile("sti; ud2");
> > 
> 
> How does the "real" triple fault occur here? Through UD2?

Yeah.  IIRC, by default L2 doesn't have a valid IDT, so any fault will escalate to
a triple fault.

> > 	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.