Re: [Xen-devel] Paravirtualization of the "HLT" instruction (for example) on x386

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

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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