[Xen-devel] [PATCH v4 3/7] x86/hap: improve hypervisor assisted guest TLB flush

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

The current implementation of the hypervisor assisted flush for HAP is
extremely inefficient.

First of all there's no need to call paging_update_cr3, as the only
relevant part of that function when doing a flush is the ASID vCPU
flush, so just call that function directly.

Since hvm_asid_flush_vcpu is protected against concurrent callers by
using atomic operations there's no need anymore to pause the affected
vCPUs.

Finally the global TLB flush performed by flush_tlb_mask is also not
necessary, since we only want to flush the guest TLB state it's enough
to trigger a vmexit on the pCPUs currently holding any vCPU state, as
such vmexit will already perform an ASID/VPID update, and thus clear
the guest TLB.

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Reviewed-by: Wei Liu <wl@xxxxxxx>
---
Changes since v3:
 - s/do_flush/handle_flush/.
 - Add comment about handle_flush usage.
 - Fix VPID typo in comment.
---
 xen/arch/x86/mm/hap/hap.c | 52 +++++++++++++++++++--------------------
 1 file changed, 25 insertions(+), 27 deletions(-)

diff --git a/xen/arch/x86/mm/hap/hap.c b/xen/arch/x86/mm/hap/hap.c
index 6894c1aa38..dbb61bf9c6 100644
--- a/xen/arch/x86/mm/hap/hap.c
+++ b/xen/arch/x86/mm/hap/hap.c
@@ -669,32 +669,28 @@ static void hap_update_cr3(struct vcpu *v, int 
do_locking, bool noflush)
     hvm_update_guest_cr3(v, noflush);
 }
 
+/*
+ * NB: doesn't actually perform any flush, used just to clear the CPU from the
+ * mask and hence signal that the guest TLB flush has been done.
+ */
+static void handle_flush(void *data)
+{
+    cpumask_t *mask = data;
+    unsigned int cpu = smp_processor_id();
+
+    ASSERT(cpumask_test_cpu(cpu, mask));
+    cpumask_clear_cpu(cpu, mask);
+}
+
 bool hap_flush_tlb(bool (*flush_vcpu)(void *ctxt, struct vcpu *v),
                    void *ctxt)
 {
     static DEFINE_PER_CPU(cpumask_t, flush_cpumask);
     cpumask_t *mask = &this_cpu(flush_cpumask);
     struct domain *d = current->domain;
+    unsigned int this_cpu = smp_processor_id();
     struct vcpu *v;
 
-    /* Avoid deadlock if more than one vcpu tries this at the same time. */
-    if ( !spin_trylock(&d->hypercall_deadlock_mutex) )
-        return false;
-
-    /* Pause all other vcpus. */
-    for_each_vcpu ( d, v )
-        if ( v != current && flush_vcpu(ctxt, v) )
-            vcpu_pause_nosync(v);
-
-    /* Now that all VCPUs are signalled to deschedule, we wait... */
-    for_each_vcpu ( d, v )
-        if ( v != current && flush_vcpu(ctxt, v) )
-            while ( !vcpu_runnable(v) && v->is_running )
-                cpu_relax();
-
-    /* All other vcpus are paused, safe to unlock now. */
-    spin_unlock(&d->hypercall_deadlock_mutex);
-
     cpumask_clear(mask);
 
     /* Flush paging-mode soft state (e.g., va->gfn cache; PAE PDPE cache). */
@@ -705,20 +701,22 @@ bool hap_flush_tlb(bool (*flush_vcpu)(void *ctxt, struct 
vcpu *v),
         if ( !flush_vcpu(ctxt, v) )
             continue;
 
-        paging_update_cr3(v, false);
+        hvm_asid_flush_vcpu(v);
 
         cpu = read_atomic(&v->dirty_cpu);
-        if ( is_vcpu_dirty_cpu(cpu) )
+        if ( cpu != this_cpu && is_vcpu_dirty_cpu(cpu) )
             __cpumask_set_cpu(cpu, mask);
     }
 
-    /* Flush TLBs on all CPUs with dirty vcpu state. */
-    flush_tlb_mask(mask);
-
-    /* Done. */
-    for_each_vcpu ( d, v )
-        if ( v != current && flush_vcpu(ctxt, v) )
-            vcpu_unpause(v);
+    /*
+     * Trigger a vmexit on all pCPUs with dirty vCPU state in order to force an
+     * ASID/VPID change and hence accomplish a guest TLB flush. Note that vCPUs
+     * not currently running will already be flushed when scheduled because of
+     * the ASID tickle done in the loop above.
+     */
+    on_selected_cpus(mask, handle_flush, mask, 0);
+    while ( !cpumask_empty(mask) )
+        cpu_relax();
 
     return true;
 }
-- 
2.25.0


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