xen/vcpu: ignore VCPU_SSHOTTMR_future

The usage of VCPU_SSHOTTMR_future in Linux prior to 4.7 is bogus.
When the hypervisor returns -ETIME (timeout in the past) Linux keeps
retrying to setup the timer with a higher timeout instead of
self-injecting a timer interrupt.

On boxes without any hardware assistance for logdirty we have seen HVM
Linux guests < 4.7 with 32vCPUs give up trying to setup the timer when
logdirty is enabled:

CE: Reprogramming failure. Giving up
CE: xen increased min_delta_ns to 1000000 nsec
CE: Reprogramming failure. Giving up
CE: Reprogramming failure. Giving up
CE: xen increased min_delta_ns to 506250 nsec
CE: xen increased min_delta_ns to 759375 nsec
CE: xen increased min_delta_ns to 1000000 nsec
CE: Reprogramming failure. Giving up
CE: Reprogramming failure. Giving up
CE: Reprogramming failure. Giving up
Freezing user space processes ...
INFO: rcu_sched detected stalls on CPUs/tasks: { 14} (detected by 10, t=60002 jiffies, g=4006, c=4005, q=14130)
Task dump for CPU 14:
swapper/14      R  running task        0     0      1 0x00000000
Call Trace:
 [<ffffffff90160f5d>] ? rcu_eqs_enter_common.isra.30+0x3d/0xf0
 [<ffffffff907b9bde>] ? default_idle+0x1e/0xd0
 [<ffffffff90039570>] ? arch_cpu_idle+0x20/0xc0
 [<ffffffff9010820a>] ? cpu_startup_entry+0x14a/0x1e0
 [<ffffffff9005d3a7>] ? start_secondary+0x1f7/0x270
 [<ffffffff900000d5>] ? start_cpu+0x5/0x14
INFO: rcu_sched detected stalls on CPUs/tasks: { 26} (detected by 24, t=60002 jiffies, g=6922, c=6921, q=7013)
Task dump for CPU 26:
swapper/26      R  running task        0     0      1 0x00000000
Call Trace:
 [<ffffffff90160f5d>] ? rcu_eqs_enter_common.isra.30+0x3d/0xf0
 [<ffffffff907b9bde>] ? default_idle+0x1e/0xd0
 [<ffffffff90039570>] ? arch_cpu_idle+0x20/0xc0
 [<ffffffff9010820a>] ? cpu_startup_entry+0x14a/0x1e0
 [<ffffffff9005d3a7>] ? start_secondary+0x1f7/0x270
 [<ffffffff900000d5>] ? start_cpu+0x5/0x14
INFO: rcu_sched detected stalls on CPUs/tasks: { 26} (detected by 24, t=60002 jiffies, g=8499, c=8498, q=7664)
Task dump for CPU 26:
swapper/26      R  running task        0     0      1 0x00000000
Call Trace:
 [<ffffffff90160f5d>] ? rcu_eqs_enter_common.isra.30+0x3d/0xf0
 [<ffffffff907b9bde>] ? default_idle+0x1e/0xd0
 [<ffffffff90039570>] ? arch_cpu_idle+0x20/0xc0
 [<ffffffff9010820a>] ? cpu_startup_entry+0x14a/0x1e0
 [<ffffffff9005d3a7>] ? start_secondary+0x1f7/0x270
 [<ffffffff900000d5>] ? start_cpu+0x5/0x14

Thus leading to CPU stalls and a broken system as a result.

Workaround this bogus usage by ignoring the VCPU_SSHOTTMR_future in
the hypervisor.  Old Linux versions are the only ones known to have
(wrongly) attempted to use the flag, and ignoring it is compatible
with the behavior expected by any guests setting that flag.

Note the usage of the flag has been removed from Linux by commit:

c06b6d70feb3 xen/x86: don't lose event interrupts

Which landed in Linux 4.7.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Henry Wang <Henry.Wang@arm.com> # CHANGELOG
Acked-by: Jan Beulich <jbeulich@suse.com>

CHANGELOG.md

@@ -9,6 +9,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 ### Changed
  - Repurpose command line gnttab_max_{maptrack_,}frames options so they don't
    cap toolstack provided values.
+ - Ignore VCPUOP_set_singleshot_timer's VCPU_SSHOTTMR_future flag. The only
+   known user doesn't use it properly, leading to in-guest breakage.
 
 ### Added
  - On x86, support for features new in Intel Sapphire Rapids CPUs: