Re: [PATCHv4 4/5] KVM: emulator: make linearize() callable from outside of emulator

[Gleb Natapov <gleb@redhat.com>]

On Thu, Jul 19, 2012 at 01:32:59PM +0300, Avi Kivity wrote:
> On 07/19/2012 10:40 AM, Gleb Natapov wrote:
> > The function will be used outside of the emulator.
> > 
> > Signed-off-by: Gleb Natapov <gleb@redhat.com>
> > ---
> >  arch/x86/include/asm/kvm_emulate.h |   16 +++++
> >  arch/x86/kvm/emulate.c             |  114 ++++++++++++++++++++----------------
> >  2 files changed, 79 insertions(+), 51 deletions(-)
> > 
> > diff --git a/arch/x86/include/asm/kvm_emulate.h b/arch/x86/include/asm/kvm_emulate.h
> > index 7c276ca..9ce651b 100644
> > --- a/arch/x86/include/asm/kvm_emulate.h
> > +++ b/arch/x86/include/asm/kvm_emulate.h
> > @@ -374,6 +374,21 @@ enum x86_intercept {
> >  	nr_x86_intercepts
> >  };
> >  
> > +struct x86_linearize_params
> > +{
> > +	enum x86emul_mode mode;
> > +	ulong ea;
> > +	unsigned size;
> > +	unsigned seg;
> > +	struct desc_struct desc;
> > +	u16 sel;
> > +	bool usable;
> > +	bool write;
> > +	bool fetch;
> > +	u8 ad_bytes;
> > +	unsigned cpl;
> > +};
> > +
> > @@ -470,14 +470,6 @@ static void set_seg_override(struct x86_emulate_ctxt *ctxt, int seg)
> >  	ctxt->seg_override = seg;
> >  }
> >  
> > -static unsigned long seg_base(struct x86_emulate_ctxt *ctxt, int seg)
> > -{
> > -	if (ctxt->mode == X86EMUL_MODE_PROT64 && seg < VCPU_SREG_FS)
> > -		return 0;
> > -
> > -	return ctxt->ops->get_cached_segment_base(ctxt, seg);
> > -}
> > -
> 
> > +int x86_linearize(struct x86_linearize_params *p, ulong *linear)
> >  {
> > -	struct desc_struct desc;
> > -	bool usable;
> >  	ulong la;
> >  	u32 lim;
> > -	u16 sel;
> >  	unsigned cpl, rpl;
> >  
> > -	la = seg_base(ctxt, addr.seg) + addr.ea;
> > -	switch (ctxt->mode) {
> > +	la = get_desc_base(&p->desc) + p->ea;
> 
> This makes 64-bit mode slower, since before the patch it avoided reading
> the segment base for non-fs/gs segments, and only read the segment base
> for fs/gs.  After the patch we always execute 4 VMREADs (and decode the
> results).
> 
That's easy to fix by making caller prepare fake desc if mode is 64-bit
and segment is non-fs/gs. The question is if this even measurable?

--
			Gleb.