Messages in this thread | | | Date | Thu, 20 Jan 2022 17:31:00 +0800 | Subject | Re: [DROP][PATCH] KVM: x86: Fix the #GP(0) and #UD conditions for XSETBV emulation | From | Xiaoyao Li <> |
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On 1/20/2022 5:17 PM, Paolo Bonzini wrote: > On 1/20/22 08:48, Like Xu wrote: >> >> In the testcase "executing XSETBV with CR4.XSAVE=0", >> >> - on the VMX, #UD delivery does not require vm-exit; > > Not your fault, it would be nicer if the Intel manual told the truth; > it says: "The following instructions cause VM exits when they are > executed in VMX non-root operation: CPUID, GETSEC[1], INVD, and XSETBV." > > Footnote [1] says "An execution of GETSEC causes an invalid-opcode > exception (#UD) if CR4.SMXE[Bit 14] = 0", and there is no such footnote > for XSETBV. Nevertheless, when tracing xsave.flat, I see that there's > a #UD vmexit and not an XSETBV vmexit: > > qemu-kvm-1637698 [019] 758186.750321: kvm_entry: > vcpu 0, rip 0x4028b7 > qemu-kvm-1637698 [019] 758186.750322: kvm_exit: > vcpu 0 reason EXCEPTION_NMI rip 0x40048d info1 0x0000000000000000 info2 > 0x0000000000000000 intr_info 0x80000306 error_code 0x00000000 > qemu-kvm-1637698 [019] 758186.750324: kvm_emulate_insn: > 0:40048d:0f 01 d1 (prot64) > qemu-kvm-1637698 [019] 758186.750325: kvm_inj_exception: #UD > (0x0) > > So while my gut feeling that #UD would not cause a vmexit was correct, > technically I was reading the SDM incorrectly.
SDM also states
Certain exceptions have priority over VM exits. These include invalid-opcode exception, faults based on privilege level, and general-protection exceptions that are based on checking I/O permission bits in the task-state segment(TSS)
in "Relative Priority of Faults and VM Exits"
So my understanding is that the architectural check always takes the higher priority than VM exit.
> Jun, can you have this fixed? > > Paolo > >> - on the SVM, #UD is trapped but goes to the ud_interception() path; >
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