Re: [patch V6 10/37] x86/entry: Switch XEN/PV hypercall entry to IDTENTRY

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

On 20/05/2020 09:06, Jürgen Groß wrote:
> On 19.05.20 21:44, Andy Lutomirski wrote:
>> On Tue, May 19, 2020 at 11:58 AM Thomas Gleixner <tglx@linutronix.de>
>> wrote:
>>>
>>> Andy Lutomirski <luto@kernel.org> writes:
>>>> B: Turn this thing around.  Specifically, in the one and only case we
>>>> care about, we know pretty much exactly what context we got this entry
>>>> in: we're running in a schedulable context doing an explicitly
>>>> preemptible hypercall, and we have RIP pointing at a SYSCALL
>>>> instruction (presumably, but we shouldn't bet on it) in the hypercall
>>>> page.  Ideally we would change the Xen PV ABI so the hypercall would
>>>> return something like EAGAIN instead of auto-restarting and we could
>>>> ditch this mess entirely.  But the ABI seems to be set in stone or at
>>>> least in molasses, so how about just:
>>>>
>>>> idt_entry(exit(regs));
>>>> if (inhcall && need_resched())
>>>>    schedule();
>>>
>>> Which brings you into the situation that you call schedule() from the
>>> point where we just moved it out. If we would go there we'd need to
>>> ensure that RCU is watching as well. idtentry_exit() might have it
>>> turned off ....
>>
>> I don't think this is possible.  Once you untangle all the wrappers,
>> the call sites are effectively:
>>
>> __this_cpu_write(xen_in_preemptible_hcall, true);
>> CALL_NOSPEC to the hypercall page
>> __this_cpu_write(xen_in_preemptible_hcall, false);
>>
>> I think IF=1 when this happens, but I won't swear to it.  RCU had
>> better be watching.
>
> Preemptible hypercalls are never done with interrupts off. To be more
> precise: they are only ever done during ioctl() processing.
>
> I can add an ASSERT() to xen_preemptible_hcall_begin() if you want.
>
>>
>> As I understand it, the one and only situation Xen wants to handle is
>> that an interrupt gets delivered during the hypercall.  The hypervisor
>> is too clever for its own good and deals with this by rewinding RIP to
>> the beginning of whatever instruction did the hypercall and delivers
>> the interrupt, and we end up in this handler.  So, if this happens,
>> the idea is to not only handle the interrupt but to schedule if
>> scheduling would be useful.
>
> Correct. More precise: the hypercalls in question can last very long
> (up to several seconds) and so they need to be interruptible. As said
> before: the interface how this is done is horrible. :-(

Forget seconds.  DOMCTL_domain_kill gets to ~14 minutes for a 2TB domain.

The reason for the existing logic is to be able voluntarily preempt.

It doesn't need to remain the way it is, but some adequate form of
pre-emption does need to stay.

~Andrew