Re: [PATCH] x86/PV: properly populate descriptor tables

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 23/09/15 16:34, Jan Beulich wrote:
> Us extending the GDT limit past the Xen descriptors so far meant that
> guests (including user mode programs) accessing any descriptor table
> slot above the original OS'es limit but below the first Xen descriptor
> caused a #PF, converted to a #GP in our #PF handler. Which is quite
> different from the native behavior, where some of such accesses (LAR
> and LSL) don't fault. Mimic that behavior by mapping a blank page into
> unused slots.
>
> While not strictly required, treat the LDT the same for consistency.
>
> Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
> Signed-off-by: Jan Beulich <jbeulich@suse.com>

Oh - thanks for doing this.  (Luckily, I hadn't yet had enough time to 
look into it.)

Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

> ---
> Not sure about 4.6 here: Beyond Andrew noticing I don't think anyone
> ran into this issue in a real world environment, and hence it doesn't
> seem to be too critical to get this fixed.

This bug causes unexpected #GP faults being handed to PV guests which 
would not occur on native, when using the `lsl` and `lsr` instructions.  
I expect it is the rarity of those instructions which is why this has 
gone unnoticed for so long.

It probably should be backported.

~Andrew