Re: [Xen-devel] Readonly GDT

[Jan Beulich]

>>> On 10.04.13 at 02:43, "H. Peter Anvin" <hpa@xxxxxxxxx> wrote:
> OK, thinking about the GDT here.
> 
> The GDT is quite small -- 256 bytes on i386, 128 bytes on x86-64.  As
> such, we probably don't want to allocate a full page to it for only
> that.  This means that in order to create a readonly mapping we have to
> pack GDTs from different CPUs together in the same pages, *or* we
> tolerate that other things on the same page gets reflected in the same
> mapping.

I think a read-only GDT is incompatible with exceptions delivered
through task gates (i.e. double fault on 32-bit), so I would assume
this needs to remain a 64-bit only thing.

> However, the packing solution has the advantage of reducing address
> space consumption which matters on 32 bits: even on i386 we can easily
> burn a megabyte of address space for 4096 processors, but burning 16
> megabytes starts to hurt.

Packing would have the additional benefit of Xen not needing to
become a special case in yet another area (because pages
containing live descriptor table entries need to be read-only for
PV guests, and need to consist of only descriptor table entries).

Jan

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