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

[Andy Lutomirski <luto@xxxxxxxxxxxxxx>]

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.

--Andy

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