Re: [Xen-devel] [PATCH 4/5] x86/pv: Drop support for paging out the LDT

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 18/01/18 10:38, George Dunlap wrote:
> On Thu, Jan 18, 2018 at 7:59 AM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
>>>>> On 12.01.18 at 19:37, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> Windows is the only OS which pages out kernel datastructures, so chances are
>>> good that this is a vestigial remnant of the PV Windows XP experiment.
>> This is based on what? How do you know there are no other OSes
>> doing so, including perhaps ones which none of us has ever heard
>> of?

The chances of there being a production OS ported to PV that we've never
head of 0.

>>  The diffstat of the change here is certainly nice, but as always
>> with something being removed from the ABI I'm rather hesitant.
>>
>>> --- a/xen/arch/x86/domain.c
>>> +++ b/xen/arch/x86/domain.c
>>> @@ -1942,11 +1942,8 @@ int domain_relinquish_resources(struct domain *d)
>>>          {
>>>              for_each_vcpu ( d, v )
>>>              {
>>> -                /*
>>> -                 * Relinquish GDT mappings. No need for explicit unmapping 
>>> of
>>> -                 * the LDT as it automatically gets squashed with the guest
>>> -                 * mappings.
>>> -                 */
>>> +                /* Relinquish GDT/LDT mappings. */
>>> +                pv_destroy_ldt(v);
>>>                  pv_destroy_gdt(v);
>> The domain is dead at this point, so the order doesn't matter much,
>> but strictly speaking you should destroy the GDT before destroying
>> the LDT (just like LDT _loads_ always need to come _after_ GDT
>> adjustments).

By that logic, I've got it the correct way round (which is they way I
intended).  Allocation and freeing of the LDT is nested within the scope
of the GDT.

>>
>> Everything else here looks fine, but the initial comment may need
>> further discussion. For example we may want to consider a
>> two-stage phasing out of the feature, with a couple of years in
>> between: Make the functionality dependent upon a default-off
>> command line option for the time being, and issue a bright warning
>> when someone actually enables it (telling them to tell us).
> One of the problems we have is that people seem to wait for 2-3 years
> after a release has been made to start updating to it.  So we'd have
> to leave such a warning for probably 5 years minimum.

In almost any other case, I'd agree, and would be the first to suggest this.

However, This patch is strictly necessary for a more complete XPTI,
which is how I stumbled onto the issue in my KAISER series to begin
with.  It is the only codepath where we ever poke at a remote critical
data structure, which is why

Also as noted in the commit message, it is broken even in the case you
wanted to sensibly page the LDT, which further backs up the exception
that it was only for Windows XP, and has never ever encountered a
production system.

~Andrew

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