Is there a way to add GDT entries without having a second page?

Is there a way to add GDT entries without having a second page?

[Kip Macy]

I can add entries incrementally to the GDT by having a second page
frame lying around copying from the current one to the unused one and
then switching between them for updates, but is there a more elegant
way?


Thanks.

    -Kip

Re: Is there a way to add GDT entries without having a second page?

[Keir Fraser]

On 6 May 2005, at 01:50, Kip Macy wrote:

> I can add entries incrementally to the GDT by having a second page
> frame lying around copying from the current one to the unused one and
> then switching between them for updates, but is there a more elegant
> way?

What's the problem? You want a 'writable GDT'? This is very doable 
since we have the emulator, but in Linux it was not hard to patch the 
places that write to the GDT to do a hypercall instead.

  -- Keir

Re: Is there a way to add GDT entries without having a second page?

[Kip Macy]

On 5/5/05, Keir Fraser <Keir.Fraser@cl.cam.ac.uk> wrote:
> 
> On 6 May 2005, at 01:50, Kip Macy wrote:
> 
> > I can add entries incrementally to the GDT by having a second page
> > frame lying around copying from the current one to the unused one and
> > then switching between them for updates, but is there a more elegant
> > way?
> 
> What's the problem? You want a 'writable GDT'? This is very doable
> since we have the emulator, but in Linux it was not hard to patch the
> places that write to the GDT to do a hypercall instead.

Each cpu in its initialization sequence adds additional entries to the
GDT. The pcpu data area is referenced through %fs, much like TLS. It
occurred to me after I had already asked the question that I could
just have the boot processor add the entries for all the CPUs so there
wouldn't be the need for any additional updates.