On Thu, Jun 4, 2015 at 3:53 PM, Tim Deegan <t...@xen.org> wrote:
> At 16:19 +0200 on 03 Jun (1433348379), Lengyel, Tamas wrote:
> > On Wed, Jun 3, 2015 at 3:48 PM, Razvan Cojocaru <
> rcojoc...@bitdefender.com>
> > wrote:
> > > On 06/03/2015 04:29 PM, Lengyel, Tamas wrote:
> > > > struct {
> > > > - uint16_t mov_to_cr0_enabled : 1;
> > > > - uint16_t mov_to_cr0_sync : 1;
> > > > - uint16_t mov_to_cr0_onchangeonly : 1;
> > > > - uint16_t mov_to_cr3_enabled : 1;
> > > > - uint16_t mov_to_cr3_sync : 1;
> > > > - uint16_t mov_to_cr3_onchangeonly : 1;
> > > > - uint16_t mov_to_cr4_enabled : 1;
> > > > - uint16_t mov_to_cr4_sync : 1;
> > > > - uint16_t mov_to_cr4_onchangeonly : 1;
> > > > + uint16_t write_ctrlreg_enabled : 4;
> > > > + uint16_t write_ctrlreg_sync : 4;
> > > > + uint16_t write_ctrlreg_onchangeonly : 4;
> > > >
> > > >
> > > > Just looking at this here again, we will now have a bitmap within a
> > > > bitmap, which doesn't seem to be very efficient. IMHO it would be
> better
> > > > to just take the ctrlreg bitmap out as a separate uint8_t within
> struct
> > > > {} monitor.
> > >
> > > How is it inefficient? I don't see that at all. And I'm not sure what
> > > you mean about the uint8_t: there are 3 fields there, each 4-bits wide
> > > (write_ctrlreg_enabled, write_ctrlreg_sync, write_ctrlreg_onchangeonly)
> > > and only (at most) two of them would fit into a uint8_t. To put them
> all
> > > into a new struct would mean wasting an uint16_t for 12-bits.
> >
> > Right now if you want to access a bit using the index on the ctrlreg
> > fields, you would for example do (monitor.write_ctrlreg_enabled & index).
> > This is actually going to perform two bitmask operations. First,
> > monitor.write_ctrlreg_enabled
> > does a masking operating to get you only the 4 bits corresponding to this
> > field, then you do another mask with the index.
>
> The compiler can optimize away the first mask operation. E.g., gcc
> folds that into a single operation at anything above -O0.
>
> Tim.
>
Good to know, thanks Tim! Sorry for the noise ;)
--
[image: www.novetta.com]
Tamas K Lengyel
Senior Security Researcher
7921 Jones Branch Drive
McLean VA 22102
Email tleng...@novetta.com
_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xen.org
http://lists.xen.org/xen-devel
