linux-kernel - Re: [PATCH] KVM: x86: fix failure of injecting exceptions inx86_emulate

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Date:   Fri, 21 Sep 2018 07:13:54 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     peng.hao2@....com.cn, sean.j.christopherson@...el.com
Cc:     wang.yi59@....com.cn, rkrcmar@...hat.com, tglx@...utronix.de,
        mingo@...hat.com, hpa@...or.com, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        zhong.weidong@....com.cn, xu.xi8@....com.cn, jiang.biao2@....com.cn
Subject: Re: [PATCH] KVM: x86: fix failure of injecting exceptions
 inx86_emulate_instruction

On 21/09/2018 04:08, peng.hao2@....com.cn wrote:
>>> Unqueued, sorry.  The hypercall test from kvm-unit-tests fails.  A
>>> VMCALL on the "edge" of canonical address space, i.e. at 0x7ffffffffffd,
>>> raises a general protection fault before this patch and a double fault
>>> afterwards.
>> Peng Hao submitted a very similar patch[1], the difference being that
>> it also modified x86_emulate_instruction() to ignore the result of
>> inject_emulated_exception().  I have no idea if that change is correct
>> but it may be related to the aforementioned unit test failing.
> The return value of inject_emulated_exception for #PF represents whether in nested vcpu, 
> not about the success or failure. Because we are not in nested vcpu, inject_emulated_exception 
> must return false. And for other exceptions,  inject_emulated_exception just return false. 
> After propagating exceptions to guest, we just should return to guest mode and let guest handle.

Yes, your patch should work, and the x86_emulate_instruction code that
is there now is clearly wrong (though I'm not sure how the double-fault
comes up, #GP + #UD shouldn't generate one because #UD is a benign
exception).

Peng Hao, do you have a testcase?  Eduardo's kvm-unit-tests patch from
Nov 2017 doesn't reproduce the bug anymore for me.

Thanks,

Paolo