linux-kernel - Re: [RFC PATCH 0/4] KVM: Emulate UMIP (or almost do so)

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Message-ID: <a0115d64-0fdc-06d8-2cd6-bbc2cb1be61d@redhat.com>
Date:   Fri, 10 Mar 2017 16:00:06 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Yu Zhang <yu.c.zhang@...ux.intel.com>
Cc:     qian.ouyang@...el.com,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [RFC PATCH 0/4] KVM: Emulate UMIP (or almost do so)

On 10/03/2017 10:31, Yu Zhang wrote:
>> We can also add a module parameter to force emulation, so that it will
>> be possible to test UMIP emulation on newer processors too.
> 
> Thanks for your reply, Paolo. :-)
> 
> Well, my previous understanding is that there might be a situation on a
> machine with UMIP
> feature:
> 1> when an APP in VM runs instructions such as sgdt addrA,
> 2> and the addrA may cause anVM exit(e.g. ept violation),
> 3> next, the emulator in hypervisor need to inject a GP fault to the VM.
> Is this situation possible?

No, the guest will execute the instruction again after the vmexit.

> But as to the scenario you described, I do not quit understand.
> I mean, on a host which do not support UMIP, although hypervisor may
> intercept cpuid and
> provide an emulated cr4 to guest, how does it guarantee those
> instructions in VM will cause a VM exit?

All instructions except SMSW can be trapped using descriptor table vmexits.

Paolo