linux-kernel - Re: [PATCH v1 13/23] KVM: VMX: Handle VMX nested exception for FRED

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Date:   Thu, 7 Dec 2023 16:42:26 +0800
From:   Chao Gao <chao.gao@...el.com>
To:     "Li, Xin3" <xin3.li@...el.com>
CC:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
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        "Shankar, Ravi V" <ravi.v.shankar@...el.com>
Subject: Re: [PATCH v1 13/23] KVM: VMX: Handle VMX nested exception for FRED

On Wed, Dec 06, 2023 at 04:37:39PM +0800, Li, Xin3 wrote:
>> Subject: RE: [PATCH v1 13/23] KVM: VMX: Handle VMX nested exception for FRED
>> 
>> > >+		if (idt_vectoring_info &
>> VECTORING_INFO_DELIVER_CODE_MASK)
>> > >+			kvm_requeue_exception_e(vcpu, vector,
>> > vmcs_read32(error_code_field),
>> > >+						idt_vectoring_info &
>> > INTR_INFO_NESTED_EXCEPTION_MASK);
>> > >+		else
>> > >+			kvm_requeue_exception(vcpu, vector,
>> > >+					      idt_vectoring_info &
>> > INTR_INFO_NESTED_EXCEPTION_MASK);
>> >
>> > Exiting-event identification can also have bit 13 set, indicating a
>> > nested exception encountered and caused VM-exit. when reinjecting the
>> > exception to guests, kvm needs to set the "nested" bit, right? I
>> > suspect some changes to e.g., handle_exception_nmi() are needed.
>> 
>> The current patch relies on kvm_multiple_exception() to do that.  But TBH, I'm
>> not sure it can recognize all nested cases.  I probably should revisit it.
>
>So the conclusion is that kvm_multiple_exception() is smart enough, and
>a VMM doesn't have to check bit 13 of the Exiting-event identification.
>
>In FRED spec 5.0, section 9.2 - New VMX Feature: VMX Nested-Exception
>Support, there is a statement at the end of Exiting-event identification:
>
>(The value of this bit is always identical to that of the valid bit of
>the original-event identification field.)
>
>It means that even w/o VMX Nested-Exception support, a VMM already knows
>if an exception is a nested exception encountered during delivery of
>another event in an exception caused VM exit (exit reason 0).  This is
>done in KVM through reading IDT_VECTORING_INFO_FIELD and calling
>vmx_complete_interrupts() immediately after VM exits.
>
>vmx_complete_interrupts() simply queues the original exception if there is
>one, and later the nested exception causing the VM exit could be cancelled
>if it is a shadow page fault.  However if the shadow page fault is caused
>by a guest page fault, KVM injects it as a nested exception to have guest
>fix its page table.
>
>I will add comments about this background in the next iteration.

is it possible that the CPU encounters an exception and causes VM-exit during
injecting an __interrupt__? in this case, no __exception__ will be (re-)queued
by vmx_complete_interrupts().