Re: [Xen-devel] [PATCH v5 2/3] x86: Remap GDT tables in the Fixmap section

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 03/09/2017 04:54 PM, Thomas Garnier wrote:
> On Thu, Mar 9, 2017 at 1:46 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> On Thu, Mar 9, 2017 at 1:43 PM, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> 
>> wrote:
>>> On 09/03/2017 21:32, Andy Lutomirski wrote:
>>>> On Mon, Mar 6, 2017 at 2:03 PM, Thomas Garnier <thgarnie@xxxxxxxxxx> wrote:
>>>>
>>>>> --- a/arch/x86/xen/enlighten.c
>>>>> +++ b/arch/x86/xen/enlighten.c
>>>>> @@ -710,7 +710,7 @@ static void load_TLS_descriptor(struct thread_struct 
>>>>> *t,
>>>>>
>>>>>         *shadow = t->tls_array[i];
>>>>>
>>>>> -       gdt = get_cpu_gdt_table(cpu);
>>>>> +       gdt = get_cpu_gdt_rw(cpu);
>>>>>         maddr = arbitrary_virt_to_machine(&gdt[GDT_ENTRY_TLS_MIN+i]);
>>>>>         mc = __xen_mc_entry(0);
>>>> Boris, is this right?  I don't see why it wouldn't be, but Xen is special.
>>> Under Xen PV, the GDT is already read-only at this point.  (It is not
>>> safe to let the guest have writeable access to system tables, so the
>>> guest must relinquish write access to the frames wishing to be used as
>>> LDTs or GDTs.)
>>>
>>> The hypercall acts on the frame, not a virtual address, so either alias
>>> should be fine here.
>>>
>>> Under this new scheme, there will be two read-only aliases.  I guess
>>> this is easier to maintain the split consistently across Linux, than to
>>> special case Xen PV because it doesn't need the second alias.
>>>
>> I think we would gain nothing at all by special-casing Xen PV -- Linux
>> allocates the fixmap vaddrs at compile time, so we'd still allocate
>> them even if we rejigger all the helpers to avoid using them.
>>
> I don't have any experience with Xen so it would be great if virtme can test 
> it.


I am pretty sure I tested this series at some point but I'll test it again.

-boris


>
> I can remove the unused functions, I just thought they were useful
> shortcuts given some of them are already used.
>
>> --Andy
>
>


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