Re: Paravirtualization of the "HLT" instruction (for example) on x386

[Anthony Liguori <aliguori@us.ibm.com>]

Mark Ryden wrote:

>>Furthermore, some instructions *have* to be paravirtualised because
>>they do not trap
>>    
>>
>
>So all the instances of such instructions which don't trap and are
>problemtaic in some sense , under the linux-xen0 and linux-xenU, are 
>replaced ?  Did I get it right ?
>  
>
Yes, a very thorough explaination of this all is available here:

http://www.cs.nps.navy.mil/people/faculty/irvine/publications/2000/VMM-usenix00-0611.pdf

Regards,

Anthony Liguori

>Mark
>
>
>
>On 1/24/06, Keir Fraser <Keir.Fraser@cl.cam.ac.uk> wrote:
>  
>
>>On 24 Jan 2006, at 11:27, Ian Brown wrote:
>>
>>    
>>
>>>I know that CLTS and WBINVD instructions, for example , should cause
>>>#GP(0) if run from CPL which is not 0; but grepping for an asm
>>>instruction
>>>which calls CLTS or WBINVD under the sparse tree gives no results.
>>>
>>>Can you give one example for such an instruction which cause a trap
>>>to the hypervisor when run in a guest OS and where in the code it
>>>causes
>>>such a trap ?
>>>
>>>(As far as I understand,if we try to issue a privilege instruction from
>>>CPL1 we should get a #GP(0) and reach the general protection
>>>handler in sparse/arch/xen/i386/kernel/traps.c ,
>>>do_general_protection().
>>>
>>>But I had looked at do_general_protection() in
>>>sparse/arch/xen/i386/kernel/traps.c
>>>and could not find there a mechanism which will trap to the
>>>hypervisor;maybe
>>>I had totally missed the point?)
>>>      
>>>
>>The main entry point for GPFs is in the hypervisor at
>>do_general_protection() in xen/arch/x86/traps.c. For certain privileged
>>instructions we perform emulation (see emulate_privileged_op() in the
>>same Xen source file). We emulate both CLTS and WBINVD for example.
>>GPFs that are not handled by Xen are indeed then passed to the guest
>>and will end up in the function you mentioned in your email.
>>
>>However, we also have paravirtualised versions of both those
>>instructions (for example, CLTS is equivalent to the hypercall
>>fpu_taskswitch(0)).
>>
>>Furthermore, some instructions *have* to be paravirtualised because
>>they do not trap (for example, POPF where the restored EFLAGS has a
>>different Interrupt-Enable flag value from current EFLAGS).
>>
>>  -- Keir
>>
>>
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>>
>>    
>>
>
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