Re: [Xen-devel] do_set_gdt

[Christian Limpach <chris@xxxxxx>]

> Given that you that the first 256 GDT entries aren't reserved and you
> explicitly state why, shouldn't the following check:
>
>     if ( (entries <= LAST_RESERVED_GDT_ENTRY) || (entries > 8192) )
>         return -EINVAL;

I think this is so that Xen can copy its GDT entries into the table which
you pass to it.  If you look in set_gdt() you'll see that Xen copies its
entries into the new GDT table.  This implies that all entries upto
FIRST_RESERVED_GDT_ENTRY have to be valid since at least
LAST_RESERVED_GDT_ENTRY many entries will have to be activated.  If you
wanted to allow a guest to request less than LAST_RESERVED_GDT_ENTRY
entries, Xen would have to (at least) copy the entries to a private GDT
table and use that.  This would still use at least 1 page so it's preferable
to just make the guest provide enough space to fit LAST_RESERVED_GDT_ENTRY
many entries and avoid a special case.

You'll also want to make sure that you don't put other stuff in the last
page which hold the GDT table and align the GDT table to a page boundary:
lgdt allows the gdt to be anywhere while for Xen you have to put it at the
beginning of a page, can't really use the rest of the last page for much
else and have to keep the pages around.

    christian



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