[Xen-devel] [PATCH v2 3/3] x86/tlb: use Xen L0 assisted TLB flush when available

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

Use Xen's L0 HVMOP_flush_tlbs hypercall in order to perform flushes.
This greatly increases the performance of TLB flushes when running
with a high amount of vCPUs as a Xen guest, and is specially important
when running in shim mode.

The following figures are from a PV guest running `make -j32 xen` in
shim mode with 32 vCPUs.

Using x2APIC and ALLBUT shorthand:
real    4m35.973s
user    4m35.110s
sys     36m24.117s

Using L0 assisted flush:
real    1m14.206s
user    4m31.835s
sys     5m45.013s

The implementation adds a new hook to hypervisor_ops so other
enlightenments can also implement such assisted flush just by filling
the hook. Note that the Xen implementation completely ignores the
dirty CPU mask and the linear address passed in, and always performs a
global TLB flush on all vCPUs.

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
Changes since v1:
 - Add a L0 assisted hook to hypervisor ops.
---
 xen/arch/x86/guest/hypervisor.c        |  9 +++++++++
 xen/arch/x86/guest/xen/xen.c           |  6 ++++++
 xen/arch/x86/smp.c                     |  6 ++++++
 xen/include/asm-x86/guest/hypervisor.h | 13 +++++++++++++
 4 files changed, 34 insertions(+)

diff --git a/xen/arch/x86/guest/hypervisor.c b/xen/arch/x86/guest/hypervisor.c
index 4f27b98740..c793ba51c2 100644
--- a/xen/arch/x86/guest/hypervisor.c
+++ b/xen/arch/x86/guest/hypervisor.c
@@ -18,6 +18,7 @@
  *
  * Copyright (c) 2019 Microsoft.
  */
+#include <xen/cpumask.h>
 #include <xen/init.h>
 #include <xen/types.h>
 
@@ -64,6 +65,14 @@ void hypervisor_resume(void)
         ops->resume();
 }
 
+int hypervisor_flush_tlb(const cpumask_t *mask, const void *va)
+{
+    if ( ops && ops->flush_tlb )
+        return ops->flush_tlb(mask, va);
+
+    return -ENOSYS;
+}
+
 /*
  * Local variables:
  * mode: C
diff --git a/xen/arch/x86/guest/xen/xen.c b/xen/arch/x86/guest/xen/xen.c
index 6dbc5f953f..47af773aca 100644
--- a/xen/arch/x86/guest/xen/xen.c
+++ b/xen/arch/x86/guest/xen/xen.c
@@ -310,11 +310,17 @@ static void resume(void)
         pv_console_init();
 }
 
+static int flush_tlb(const cpumask_t *mask, const void *va)
+{
+    return xen_hypercall_hvm_op(HVMOP_flush_tlbs, NULL);
+}
+
 static const struct hypervisor_ops ops = {
     .name = "Xen",
     .setup = setup,
     .ap_setup = ap_setup,
     .resume = resume,
+    .flush_tlb = flush_tlb,
 };
 
 const struct hypervisor_ops *__init xg_probe(void)
diff --git a/xen/arch/x86/smp.c b/xen/arch/x86/smp.c
index 6510dd84ab..b3cb1ba90f 100644
--- a/xen/arch/x86/smp.c
+++ b/xen/arch/x86/smp.c
@@ -15,6 +15,7 @@
 #include <xen/perfc.h>
 #include <xen/spinlock.h>
 #include <asm/current.h>
+#include <asm/guest.h>
 #include <asm/smp.h>
 #include <asm/mc146818rtc.h>
 #include <asm/flushtlb.h>
@@ -262,6 +263,11 @@ void flush_area_mask(const cpumask_t *mask, const void 
*va, unsigned int flags)
     if ( (flags & ~FLUSH_ORDER_MASK) &&
          !cpumask_subset(mask, cpumask_of(cpu)) )
     {
+        if ( cpu_has_hypervisor &&
+             !(flags & ~(FLUSH_TLB | FLUSH_TLB_GLOBAL | FLUSH_VA_VALID)) &&
+             !hypervisor_flush_tlb(mask, va) )
+            return;
+
         spin_lock(&flush_lock);
         cpumask_and(&flush_cpumask, mask, &cpu_online_map);
         cpumask_clear_cpu(cpu, &flush_cpumask);
diff --git a/xen/include/asm-x86/guest/hypervisor.h 
b/xen/include/asm-x86/guest/hypervisor.h
index 392f4b90ae..b7f1969796 100644
--- a/xen/include/asm-x86/guest/hypervisor.h
+++ b/xen/include/asm-x86/guest/hypervisor.h
@@ -28,6 +28,8 @@ struct hypervisor_ops {
     void (*ap_setup)(void);
     /* Resume from suspension */
     void (*resume)(void);
+    /* L0 assisted TLB flush */
+    int (*flush_tlb)(const cpumask_t *mask, const void *va);
 };
 
 #ifdef CONFIG_GUEST
@@ -36,9 +38,16 @@ const char *hypervisor_probe(void);
 void hypervisor_setup(void);
 void hypervisor_ap_setup(void);
 void hypervisor_resume(void);
+/*
+ * L0 assisted TLB flush.
+ * mask: cpumask of the dirty vCPUs that should be flushed.
+ * va: linear address to flush, or NULL for global flushes.
+ */
+int hypervisor_flush_tlb(const cpumask_t *mask, const void *va);
 
 #else
 
+#include <xen/cpumask.h>
 #include <xen/lib.h>
 #include <xen/types.h>
 
@@ -46,6 +55,10 @@ static inline const char *hypervisor_probe(void) { return 
NULL; }
 static inline void hypervisor_setup(void) { ASSERT_UNREACHABLE(); }
 static inline void hypervisor_ap_setup(void) { ASSERT_UNREACHABLE(); }
 static inline void hypervisor_resume(void) { ASSERT_UNREACHABLE(); }
+static inline int hypervisor_flush_tlb(const cpumask_t *mask, const void *va)
+{
+    return -ENOSYS;
+}
 
 #endif  /* CONFIG_GUEST */
 
-- 
2.24.1


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