[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] do_set_gdt
> 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 ------------------------------------------------------- SF.Net is sponsored by: Speed Start Your Linux Apps Now. Build and deploy apps & Web services for Linux with a free DVD software kit from IBM. Click Now! http://ads.osdn.com/?ad_id=1356&alloc_id=3438&op=click _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.sourceforge.net/lists/listinfo/xen-devel
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