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 10:49:00 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Xiaoyao Li <xiaoyao.li@...el.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 1/20/22 10:31, Xiaoyao Li wrote:
>>
>> 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.

Good point!  It's right above in 25.1.1.  I was confused by the specific 
mention of GETSEC, but the reason for the footnote is because undefined 
GETSEC leaves cause a vmexit instead of #UD, and GETSEC vmexits also 
override #GP faults based on privilege level.

Thanks,

Paolo