[RFC PATCH v7 31/31] x86/entry: Add an option to coalesce TLB flushes

[Valentin Schneider <vschneid@redhat.com>]

Previous patches have introduced a mechanism to prevent kernel text updates
from inducing interference on isolated CPUs. A similar action is required
for kernel-range TLB flushes in order to silence the biggest remaining
cause of isolated CPU IPI interference.

These flushes are mostly caused by vmalloc manipulations - e.g. on x86 with
CONFIG_VMAP_STACK, spawning enough processes will easily trigger
flushes. Unfortunately, the newly added context_tracking IPI deferral
mechanism cannot be leveraged for TLB flushes, as the deferred work would
be executed too late. Consider the following execution flow:

  <userspace>

  !interrupt!

  SWITCH_TO_KERNEL_CR3 // vmalloc range becomes accessible

  idtentry_func_foo()
    irqentry_enter()
      irqentry_enter_from_user_mode()
	enter_from_user_mode()
	  [...]
	    ct_kernel_enter_state()
	      ct_work_flush() // deferred flush would be done here

Since there is no sane way to assert no stale entry is accessed during
kernel entry, any code executed between SWITCH_TO_KERNEL_CR3 and
ct_work_flush() is at risk of accessing a stale entry. Dave had suggested
hacking up something within SWITCH_TO_KERNEL_CR3 itself, which is what has
been implemented in the previous patches.

Make kernel-range TLB flush deferral available via CONFIG_COALESCE_TLBI.

Signed-off-by: Valentin Schneider <vschneid@redhat.com>
---
 arch/x86/Kconfig | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index fa9229c0e0939..04f9d6496bbbc 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2189,6 +2189,23 @@ config ADDRESS_MASKING
	  The capability can be used for efficient address sanitizers (ASAN)
	  implementation and for optimizations in JITs.

+config COALESCE_TLBI
+       def_bool n
+       prompt "Coalesce kernel TLB flushes for NOHZ-full CPUs"
+       depends on X86_64 && MITIGATION_PAGE_TABLE_ISOLATION && NO_HZ_FULL
+       help
+	 TLB flushes for kernel addresses can lead to IPIs being sent to
+	 NOHZ-full CPUs, thus kicking them out of userspace.
+
+	 This option coalesces kernel-range TLB flushes for NOHZ-full CPUs into
+	 a single flush executed at kernel entry, right after switching to the
+	 kernel page table. Note that this flush is unconditionnal, even if no
+	 remote flush was issued during the previous userspace execution window.
+
+	 This obviously makes the user->kernel transition overhead even worse.
+
+	 If unsure, say N.
+
 config HOTPLUG_CPU
	def_bool y
	depends on SMP
--
2.51.0