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

[Gleb Natapov <gleb@redhat.com>]

On Thu, Jul 19, 2012 at 03:52:15PM +0300, Avi Kivity wrote:
> On 07/19/2012 01:51 PM, Gleb Natapov wrote:
> 
> >> > +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?
> 
> I'm sure it will be measurable, esp. on older processors.  Why not
> measure it?
> 
> 
It is easier to just fix it :) Will do and resend if you are agree with
general approach.

--
			Gleb.