From: "Li, Xin3" <xin3.li@intel.com>
To: "Gao, Chao" <chao.gao@intel.com>
Cc: "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
"linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
"linux-kselftest@vger.kernel.org"
<linux-kselftest@vger.kernel.org>,
"seanjc@google.com" <seanjc@google.com>,
"pbonzini@redhat.com" <pbonzini@redhat.com>,
"corbet@lwn.net" <corbet@lwn.net>,
"kys@microsoft.com" <kys@microsoft.com>,
"haiyangz@microsoft.com" <haiyangz@microsoft.com>,
"wei.liu@kernel.org" <wei.liu@kernel.org>,
"Cui, Dexuan" <decui@microsoft.com>,
"tglx@linutronix.de" <tglx@linutronix.de>,
"mingo@redhat.com" <mingo@redhat.com>,
"bp@alien8.de" <bp@alien8.de>,
"dave.hansen@linux.intel.com" <dave.hansen@linux.intel.com>,
"x86@kernel.org" <x86@kernel.org>,
"hpa@zytor.com" <hpa@zytor.com>,
"vkuznets@redhat.com" <vkuznets@redhat.com>,
"peterz@infradead.org" <peterz@infradead.org>,
"Shankar, Ravi V" <ravi.v.shankar@intel.com>
Subject: RE: [PATCH v1 13/23] KVM: VMX: Handle VMX nested exception for FRED
Date: Fri, 8 Dec 2023 23:48:37 +0000 [thread overview]
Message-ID: <SA1PR11MB6734FFE3D3500BFE4AF407DDA88AA@SA1PR11MB6734.namprd11.prod.outlook.com> (raw)
In-Reply-To: <ZXJ3z0yCqnHm52sC@chao-email>
> >> >> > 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().
> >
> >I guess the following case is what you're suggesting:
> >KVM injects an external interrupt after shadow page tables are nuked.
> >
> >vmx_complete_interrupts() are called after each VM exit to clear both
> >interrupt and exception queues, which means it always pushes the
> >deepest event if there is an original event. In the above case, the
> >original event is the external interrupt KVM just tried to inject.
>
> in my understanding, your point is:
> 1. if bit 13 of the Exiting-event identification is set. the original-event
> identification field should be valid.
> 2. vmx_complete_interrupts() is done immediately after VM exits and reads
> original-event identification and reinjects the event there.
> 3. if KVM injects the exception in exiting-event identification
> to guest, KVM doesn't need to read the bit 13 because kvm_multiple_exception()
> is "smart enough" and recognize the exception as nested-exception because if
> bit 13 is 1, one exception must has been queued in #2.
>
> my question is:
> what if the event in original-event identification is an interrupt e.g.,
> external interrupt or NMI, rather than exception. vmx_complete_interrupts()
> won't queue an exception, then how can KVM or kvm_multiple_exception()
> know the
> exception that caused VM-exit is an nested exception w/o reading bit 13 of the
> Exiting-event identification?
The good news is that vmx_complete_interrupts() still queues the event
even it's not a hardware exception. It's just that kvm_multiple_exception()
doesn't check if there is an original interrupt or NMI because IDT event
delivery doesn't care such a case.
I think your point is more of that we should check it when FRED is enabled
for a guest. Yes, architecturally we should do it.
What I want to emphasize is that bit 13 of the exiting-event identification
is set to the valid bit of the original-event identification, they are
logically the same thing when FRED is enabled. It doens't matter which one
a VMM reads and uses. But a VMM doesn't need to differentiate FRED and IDT
if it reads the info from original-event identification.
next prev parent reply other threads:[~2023-12-08 23:48 UTC|newest]
Thread overview: 70+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-11-08 18:29 [PATCH v1 00/23] Enable FRED with KVM VMX Xin Li
2023-11-08 18:29 ` [PATCH v1 01/23] KVM: VMX: Cleanup VMX basic information defines and usages Xin Li
2023-11-08 18:29 ` [PATCH v1 02/23] KVM: VMX: Cleanup VMX misc " Xin Li
2023-11-08 18:29 ` [PATCH v1 03/23] KVM: VMX: Add support for the secondary VM exit controls Xin Li
2023-11-08 18:29 ` [PATCH v1 04/23] KVM: x86: Mark CR4.FRED as not reserved Xin Li
2023-11-08 18:29 ` [PATCH v1 05/23] KVM: VMX: Initialize FRED VM entry/exit controls in vmcs_config Xin Li
2023-11-09 8:53 ` Chao Gao
2023-11-09 15:15 ` Sean Christopherson
2023-11-10 0:04 ` Li, Xin3
2023-11-10 15:01 ` Sean Christopherson
2023-11-14 4:05 ` Li, Xin3
2023-11-13 17:18 ` Nikolay Borisov
2023-11-15 2:39 ` Li, Xin3
2023-11-08 18:29 ` [PATCH v1 06/23] KVM: VMX: Defer enabling FRED MSRs save/load until after set CPUID Xin Li
2023-11-09 9:15 ` Chao Gao
2023-11-09 23:50 ` Li, Xin3
2023-11-10 0:18 ` Sean Christopherson
2023-11-14 2:50 ` Li, Xin3
2023-11-15 21:47 ` Sean Christopherson
2023-11-08 18:29 ` [PATCH v1 07/23] KVM: VMX: Disable intercepting FRED MSRs Xin Li
2023-11-09 9:21 ` Chao Gao
2023-11-08 18:29 ` [PATCH v1 08/23] KVM: VMX: Initialize VMCS FRED fields Xin Li
2023-11-13 3:04 ` Chao Gao
2023-11-14 6:02 ` Li, Xin3
2023-11-14 6:51 ` Li, Xin3
2023-11-08 18:29 ` [PATCH v1 09/23] KVM: VMX: Switch FRED RSP0 between host and guest Xin Li
2023-11-13 3:47 ` Chao Gao
2023-11-14 5:17 ` Li, Xin3
2023-11-14 7:47 ` Chao Gao
2023-11-15 3:04 ` Li, Xin3
2023-11-08 18:29 ` [PATCH v1 10/23] KVM: VMX: Add support for FRED context save/restore Xin Li
2023-11-13 5:24 ` Chao Gao
2023-11-14 4:48 ` Li, Xin3
2023-11-08 18:29 ` [PATCH v1 11/23] KVM: x86: Add kvm_is_fred_enabled() Xin Li
2023-11-13 7:35 ` Chao Gao
2023-11-14 4:42 ` Li, Xin3
2023-11-14 8:16 ` Chao Gao
2023-11-14 18:57 ` Li, Xin3
2023-11-20 9:04 ` Li, Xin3
2023-11-08 18:29 ` [PATCH v1 12/23] KVM: VMX: Handle FRED event data Xin Li
2023-11-13 10:14 ` Chao Gao
2023-11-14 4:34 ` Li, Xin3
2023-11-14 8:58 ` Chao Gao
2023-11-15 2:52 ` Li, Xin3
2023-11-16 2:39 ` Chao Gao
2023-11-20 8:16 ` Li, Xin3
2023-11-08 18:29 ` [PATCH v1 13/23] KVM: VMX: Handle VMX nested exception for FRED Xin Li
2023-11-14 7:40 ` Chao Gao
2023-11-15 3:03 ` Li, Xin3
2023-12-06 8:37 ` Li, Xin3
2023-12-07 8:42 ` Chao Gao
2023-12-07 10:09 ` Li, Xin3
2023-12-08 1:56 ` Chao Gao
2023-12-08 23:48 ` Li, Xin3 [this message]
2023-11-08 18:29 ` [PATCH v1 14/23] KVM: VMX: Dump FRED context in dump_vmcs() Xin Li
2023-11-14 14:36 ` Nikolay Borisov
2023-11-15 2:41 ` Li, Xin3
2023-11-08 18:29 ` [PATCH v1 15/23] KVM: nVMX: Add support for the secondary VM exit controls Xin Li
2023-11-09 8:21 ` Jeremi Piotrowski
2023-11-10 0:12 ` Li, Xin3
2023-11-20 15:52 ` Vitaly Kuznetsov
2023-11-20 17:42 ` Li, Xin3
2023-11-08 18:29 ` [PATCH v1 16/23] KVM: nVMX: Add FRED VMCS fields Xin Li
2023-11-08 18:29 ` [PATCH v1 17/23] KVM: nVMX: Add support for VMX FRED controls Xin Li
2023-11-08 18:29 ` [PATCH v1 18/23] KVM: nVMX: Add VMCS FRED states checking Xin Li
2023-11-08 18:29 ` [PATCH v1 19/23] KVM: x86: Allow FRED/LKGS/WRMSRNS to be exposed to guests Xin Li
2023-11-08 18:30 ` [PATCH v1 20/23] KVM: selftests: Add FRED VMCS fields to evmcs Xin Li
2023-11-08 18:30 ` [PATCH v1 21/23] KVM: selftests: Run debug_regs test with FRED enabled Xin Li
2023-11-08 18:30 ` [PATCH v1 22/23] KVM: selftests: Add a new VM guest mode to run user level code Xin Li
2023-11-08 18:30 ` [PATCH v1 23/23] KVM: selftests: Add fred exception tests Xin Li
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