Re: [Xen-devel] [PATCH v3 2/2] vmx/hap: optimize CR4 trapping

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Sat, Feb 24, 2018 at 01:47:37AM +0000, Tian, Kevin wrote:
> > From: Roger Pau Monné [mailto:roger.pau@xxxxxxxxxx]
> > Sent: Friday, February 23, 2018 6:18 PM
> > 
> > On Fri, Feb 23, 2018 at 04:56:38AM +0000, Tian, Kevin wrote:
> > > > From: Roger Pau Monne [mailto:roger.pau@xxxxxxxxxx]
> > > > +        __vmwrite(CR4_GUEST_HOST_MASK, v-
> > > > >arch.hvm_vcpu.cr4_host_mask);
> > > > +
> > >
> > > what about doing a comparison to avoid vmwrite if nothing changed?
> > 
> > I can do that, but it doesn't seem like other writes in the same
> > function do this (see GUEST_CR4 unconditional write above for
> > example).
> > 
> > Is it really better from a performance PoV to avoid the write? (or
> > take the penalty of doing a read + write?)
> > 
> 
> not much. just personal favorite. :-)
> 
> Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>

The AMD guys requested some changes so I had to slightly change the
patch and didn't keep you ack. Could you please take a look at:

https://marc.info/?l=xen-devel&m=151963964317894

Thanks, Roger.

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