linux-kernel - Re: [PATCH v1 12/23] KVM: VMX: Handle FRED event data

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Date:   Tue, 14 Nov 2023 16:58:59 +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>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
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        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "corbet@....net" <corbet@....net>,
        "kys@...rosoft.com" <kys@...rosoft.com>,
        "haiyangz@...rosoft.com" <haiyangz@...rosoft.com>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>,
        "Cui, Dexuan" <decui@...rosoft.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
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        "vkuznets@...hat.com" <vkuznets@...hat.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>
Subject: Re: [PATCH v1 12/23] KVM: VMX: Handle FRED event data

On Tue, Nov 14, 2023 at 12:34:02PM +0800, Li, Xin3 wrote:
>> >+			else if (is_nm_fault(intr_info) &&
>> >+				 vcpu->arch.guest_fpu.fpstate->xfd)
>> 
>> does this necessarily mean the #NM is caused by XFD?
>
>Then the event data should be 0.  Or I missed something obvious? I.e.,
>it can be easily differentiated and we should just explicitly set it
>to 0.

vcpu->arch.guest_fpu.fpstate->xfd just means the guest is enabling XFD.
I don't think we can conclude that this #NM is caused by XFD only from
this.  i.e., there may be some false positives.

>> >+			u64 event_data = vmcs_read64(event_data_field);
>> >+
>> >+			switch (vector) {
>> >+			case DB_VECTOR:
>> >+				get_debugreg(vcpu->arch.dr6, 6);
>> >+				WARN_ON(vcpu->arch.dr6 != (event_data ^
>> DR6_RESERVED));
>> >+				vcpu->arch.dr6 = event_data ^ DR6_RESERVED;
>> >+				break;
>> >+			case NM_VECTOR:
>> >+				if (vcpu->arch.guest_fpu.fpstate->xfd) {
>> >+					rdmsrl(MSR_IA32_XFD_ERR, vcpu-
>> >arch.guest_fpu.xfd_err);
>> >+					WARN_ON(vcpu-
>> >arch.guest_fpu.xfd_err != event_data);
>> >+					vcpu->arch.guest_fpu.xfd_err =
>> event_data;
>> >+				} else {
>> >+					WARN_ON(event_data != 0);
>> >+				}
>> >+				break;
>> >+			case PF_VECTOR:
>> >+				WARN_ON(vcpu->arch.cr2 != event_data);
>> >+				vcpu->arch.cr2 = event_data;
>> >+				break;
>> >+			default:
>> >+				WARN_ON(event_data != 0);
>> 
>> I am not sure if this WARN_ON() can be triggeded by nested VMX. It is legitimate
>> for L1 VMM to inject any event w/ an event_data.
>> 
>> FRED spec says:
>> 
>> Section 5.2.1 specifies the event data that FRED event delivery of certain events
>> saves on the stack. When FRED event delivery is used for an event injected by VM
>> entry, the event data saved is the value of the injected-event-data field in the
>> VMCS. This value is used instead of what is specified in Section 5.2.1 and is done
>> for __ALL__ injected events using FRED event delivery
>
>5.2.1 Saving Information on the Regular Stack also says:
>- For any other event, the event data are not currently defined and will
>  be zero until they are.
>
>Or you mean something else?

IIUC, L1 KVM can inject a nested exception whose vector isn't #DB, or #NM or
#PF with a non-zero event_data to L2. If delivering the nested exception causes a
VM-exit to L0 KVM, the assertion that event_data is always 0 for vectors other
than #DB/#NM/#PF fails.