From mboxrd@z Thu Jan  1 00:00:00 1970
From: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Subject: Re: [PATCH v2 1/8] KVM: Fix load_guest_segment_descriptor() to inject
 page fault
Date: Fri, 12 Feb 2010 09:13:50 +0900
Message-ID: <4B749D3E.8080207@oss.ntt.co.jp>
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: mtosatti@redhat.com, kvm@vger.kernel.org
To: Avi Kivity <avi@redhat.com>
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Sorry for being late to reply.
   Japan was a holiday yesterday.

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?
> 


I did not notice about that, we need not inject page faults for them.
But as Marcelo says in the next mail ...