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

To: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Subject: Re: [Xen-devel] Paravirtualization of the "HLT" instruction (for example) on x386
From: Mark Ryden <markryde@xxxxxxxxx>
Date: Sat, 28 Jan 2006 17:41:10 +0200
Cc: Ian Brown <ianbrn@xxxxxxxxx>, Xen Mailing List <xen-devel@xxxxxxxxxxxxxxxxxxx>
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>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 ?


On 1/24/06, Keir Fraser <Keir.Fraser@xxxxxxxxxxxx> 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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