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Re: [Xen-devel] [PATCH v3 1/2] xen/PVH: Set up GS segment for stack canary

>>> On 17.05.18 at 19:47, <boris.ostrovsky@xxxxxxxxxx> wrote:
> On 05/17/2018 11:02 AM, Jan Beulich wrote:
>>>>> On 17.05.18 at 16:47, <boris.ostrovsky@xxxxxxxxxx> wrote:
>>> @@ -64,6 +67,9 @@ ENTRY(pvh_start_xen)
>>>     mov %eax,%es
>>>     mov %eax,%ss
>>> +   mov $PVH_CANARY_SEL,%eax
>>> +   mov %eax,%gs
>> I doubt this is needed for 64-bit (you could equally well load zero or leave
>> in place what's there in that case),
> I don't understand this.

The actual selector value doesn't matter on 64-bit. Hence you could
omit the load altogether, or you could use plain zero. No need for the
(non-zero) selector, or (by implication) the GDT descriptor.

>>  and loading the selector before setting
>> the base address in the descriptor won't have the intended effect.
> I wasn't sure about this either but then I noticed that
> secondary_startup_64() does it in the same order (although not using the
> MSR).

Well, for one they load a null selector, which is independent of setting up
any GDT descriptors. I also don't understand why you say "although not
using the MSR" when they clearly do. And then, as said above (and also
in a comment in secondary_startup_64()), the actual selector value (and
when / if at all it is loaded) doesn't matter on 64-bit. The ordering does
matter on 32-bit though.


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