linux-kernel - Re: [DROP][PATCH] KVM: x86: Fix the #GP(0) and #UD conditions for XSETBV emulation

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Date:   Thu, 20 Jan 2022 17:31:00 +0800
From:   Xiaoyao Li <xiaoyao.li@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Like Xu <like.xu.linux@...il.com>,
        Jun Nakajima <jun.nakajima@...el.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [DROP][PATCH] KVM: x86: Fix the #GP(0) and #UD conditions for
 XSETBV emulation

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;
>