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Date: Fri, 17 Jul 2015 12:48:41 -0400
From: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
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Subject: Re: [Qemu-devel] [PATCH v1 05/10] xen/pt: Remove XenPTReg->data
	field.
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To: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: xen-devel@lists.xenproject.org, qemu-devel@nongnu.org, JBeulich@suse.com

On Fri, Jul 17, 2015 at 05:30:39PM +0100, Stefano Stabellini wrote:
> On Thu, 2 Jul 2015, Konrad Rzeszutek Wilk wrote:
> > We do not want to have two entries to cache the guest configuration
> > registers: XenPTReg->data and dev.config. Instead we want to use
> > only the dev.config.
> > 
> > To do without much complications we rip out the ->data field
> > and replace it with an pointer to the dev.config. This way we
> > have the type-checking (uint8_t, uint16_t, etc) and as well
> > and pre-computed location.
> > 
> > Alternatively we could compute the offset in dev.config by
> > using the XenPTRRegInfo and XenPTRegGroup every time but
> > this way we have the pre-computed values.
> > 
> > Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> > ---
> >  hw/xen/xen_pt.h             |  6 +++-
> >  hw/xen/xen_pt_config_init.c | 73 +++++++++++++++++++++++++++------------------
> >  2 files changed, 49 insertions(+), 30 deletions(-)
> > 
> > diff --git a/hw/xen/xen_pt.h b/hw/xen/xen_pt.h
> > index 09358b1..586d055 100644
> > --- a/hw/xen/xen_pt.h
> > +++ b/hw/xen/xen_pt.h
> > @@ -134,7 +134,11 @@ struct XenPTRegInfo {
> >  struct XenPTReg {
> >      QLIST_ENTRY(XenPTReg) entries;
> >      XenPTRegInfo *reg;
> > -    uint32_t data; /* emulated value */
> > +    union {
> > +        uint8_t *byte;
> > +        uint16_t *word;
> > +        uint32_t *dbword;
> > +    } ptr; /* pointer to dev.config. */
> 
> Nice (nasty?) trick. I would probably have just introduced a single
> pointer, but this might turn out to be better as it avoids the risk of
> merging bits that should be ignored.

Right, and also adds nice type-casting checks.
> 
> However it should be half-word (uint16_t) and word (uint32_t).

OK.