From mboxrd@z Thu Jan  1 00:00:00 1970
From: Marcelo Tosatti <mtosatti@redhat.com>
Subject: Re: [PATCH v2 1/8] KVM: Fix load_guest_segment_descriptor() to
 inject page fault
Date: Wed, 10 Feb 2010 14:29:23 -0200
Message-ID: <20100210162923.GA24073@amt.cnet>
References: <20100210104541.e0f1fdd1.yoshikawa.takuya@oss.ntt.co.jp>
 <20100210105034.21644f5b.yoshikawa.takuya@oss.ntt.co.jp>
 <4B72DE06.3020909@redhat.com>
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Cc: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>,
	kvm@vger.kernel.org
To: Avi Kivity <avi@redhat.com>
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On Wed, Feb 10, 2010 at 06:25:42PM +0200, Avi Kivity wrote:
> On 02/10/2010 03:50 AM, Takuya Yoshikawa wrote:
> >This patch injects page fault when reading descriptor in
> >load_guest_segment_descriptor() fails with FAULT.
> >
> >Effects of this injection: This function is used by
> >kvm_load_segment_descriptor() which is necessary for the
> >following instructions.
> >  - mov seg,r/m16
> >  - jmp far
> >  - pop ?s
> >This patch makes it possible to emulate the page faults
> >generated by these instructions. But be sure that unless
> >we change the kvm_load_segment_descriptor()'s ret value
> >propagation this patch has no effect.
> >
> >
> >@@ -4655,6 +4655,7 @@ static int load_guest_segment_descriptor(struct kvm_vcpu *vcpu, u16 selector,
> >  {
> >  	struct descriptor_table dtable;
> >  	u16 index = selector>>  3;
> >+	int ret;
> >
> >  	get_segment_descriptor_dtable(vcpu, selector,&dtable);
> >
> >@@ -4662,7 +4663,11 @@ static int load_guest_segment_descriptor(struct kvm_vcpu *vcpu, u16 selector,
> >  		kvm_queue_exception_e(vcpu, GP_VECTOR, selector&  0xfffc);
> >  		return X86EMUL_PROPAGATE_FAULT;
> >  	}
> >-	return kvm_read_guest_virt(dtable.base + index*8, seg_desc, sizeof(*seg_desc), vcpu);
> >+	ret = kvm_read_guest_virt(dtable.base + index*8, seg_desc, sizeof(*seg_desc), vcpu);
> >+	if (ret == X86EMUL_PROPAGATE_FAULT)
> >+		kvm_inject_page_fault(vcpu, dtable.base + index*8, 0);
> >+
> >+	return ret;
> >  }
> 
> If the descriptor table is not aligned, and a descriptor spans two
> pages, then we might need to inject a page fault at some other
> address.
> 
> Also, the injection should be done in kvm_read_guest_virt() to avoid
> duplicating code.
> 
> These instructions however are only emulated in big real mode.
> Where did you encounter the need to inject page faults during their
> emulation?

Task switch exits also use them, so it should inject faults as
documented.