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[Xen-devel] [PATCH v5 4/4] x86/smp: do not use scratch_cpumask when in interrupt or exception context

Using scratch_cpumask in send_IPI_mask is not safe in IRQ or exception
context because it can nest, and hence send_IPI_mask could be
overwriting another user scratch cpumask data when used in such

Instead introduce a new cpumask to be used by send_IPI_mask, and
disable interrupts while using it.

Fallback to not using the scratch cpumask (and hence not attemping to
optimize IPI sending by using a shorthand) when in IRQ or exception
context. Note that the scratch cpumask cannot be used when
non-maskable interrupts are being serviced (NMI or #MC) and hence
fallback to not using the shorthand in that case, like it was done

Fixes: 5500d265a2a8 ('x86/smp: use APIC ALLBUT destination shorthand when 
Reported-by: Sander Eikelenboom <linux@xxxxxxxxxxxxxx>
Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Changes since v4:
 - Add _handler suffix to in_nmi/in_mce calls.

Changes since v3:
 - Do not use a dedicated cpumask, and instead prevent usage when in
   IRQ context.

Changes since v2:
 - Fallback to the previous IPI sending mechanism in #MC or #NMI

Changes since v1:
 - Don't use the shorthand when in #MC or #NMI context.
 xen/arch/x86/smp.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/xen/arch/x86/smp.c b/xen/arch/x86/smp.c
index 55d08c9d52..0461812cf6 100644
--- a/xen/arch/x86/smp.c
+++ b/xen/arch/x86/smp.c
@@ -69,6 +69,18 @@ void send_IPI_mask(const cpumask_t *mask, int vector)
     bool cpus_locked = false;
     cpumask_t *scratch = this_cpu(scratch_cpumask);
+    if ( in_irq() || in_mce_handler() || in_nmi_handler() )
+    {
+        /*
+         * When in IRQ, NMI or #MC context fallback to the old (and simpler)
+         * IPI sending routine, and avoid doing any performance optimizations
+         * (like using a shorthand) in order to avoid using the scratch
+         * cpumask which cannot be used in interrupt context.
+         */
+        alternative_vcall(genapic.send_IPI_mask, mask, vector);
+        return;
+    }
      * This can only be safely used when no CPU hotplug or unplug operations
      * are taking place, there are no offline CPUs (unless those have been

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