linux-kernel - Re: [RFC PATCH 3/4] KVM: x86: add support for emulating UMIP

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Message-ID: <af995640-7f54-18ea-0589-f04d8ec9b054@redhat.com>
Date:	Thu, 14 Jul 2016 09:00:30 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Radim Krčmář <rkrcmar@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [RFC PATCH 3/4] KVM: x86: add support for emulating UMIP



On 13/07/2016 22:03, Radim Krčmář wrote:
> 2016-07-12 21:20+0200, Paolo Bonzini:
>> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
>> ---
>> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
>> @@ -3738,6 +3747,10 @@ static int em_lidt(struct x86_emulate_ctxt *ctxt)
>>  
>>  static int em_smsw(struct x86_emulate_ctxt *ctxt)
>>  {
>> +	if ((ctxt->ops->get_cr(ctxt, 4) & X86_CR4_UMIP) &&
>> +	    ctxt->ops->cpl(ctxt) > 0)
> 
> UMIP should #GP(0) in virtual-8086 mode too (for SMSW, SIDT, and SGDT),
> but cpl() returns 0 in vm86.

No, it returns 3.

vmx->rmode.vm86_active means "there is no unrestricted guest support,
and we're using VM86 mode to emulate real mode".  The special case in
vmx_get_cpl is needed exactly because real mode CPL is 0 but VM86 has
CPL 3.  In fact fix_rmode_seg sets DPL=3 for all segments, and CPL=SS.DPL.

Paolo

>> +		return emulate_gp(ctxt, 0);
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