From: Roger Pau Monné <roger.pau@citrix.com>
Date: Mon, 4 May 2020 09:53:01 +0000 (+0200)
Subject: x86/hap: be more selective with assisted TLB flush
X-Git-Tag: archive/raspbian/4.14.0+80-gd101b417b7-1+rpi1^2~63^2~314
X-Git-Url: https://dgit.raspbian.org/?a=commitdiff_plain;h=17b997aa1edb9eb8d9bd1958457ff50927f46832;p=xen.git

x86/hap: be more selective with assisted TLB flush

When doing an assisted flush on HAP the purpose of the
on_selected_cpus is just to trigger a vmexit on remote CPUs that are
in guest context, and hence just using is_vcpu_dirty_cpu is too lax,
also check that the vCPU is running. Due to the lazy context switching
done by Xen dirty_cpu won't always be cleared when the guest vCPU is
not running, and hence relying on is_running allows more fine grained
control of whether the vCPU is actually running.

I've measured the time of the non-local branch of flush_area_mask
inside the shim running with 32vCPUs over 100000 executions and
averaged the result on a large Westmere system (80 ways total). The
figures where fetched during the boot of a SLES 11 PV guest. The
results are as follow (less is better):

Non assisted flush with x2APIC:      112406ns
Assisted flush without this patch:   820450ns
Assisted flush with this patch:        8330ns

While there also pass NULL as the data parameter of on_selected_cpus,
the dummy handler doesn't consume the data in any way.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
---

diff --git a/xen/arch/x86/mm/hap/hap.c b/xen/arch/x86/mm/hap/hap.c
index 580d1c2164..0275cdf5c8 100644
--- a/xen/arch/x86/mm/hap/hap.c
+++ b/xen/arch/x86/mm/hap/hap.c
@@ -719,7 +719,7 @@ static bool flush_tlb(bool (*flush_vcpu)(void *ctxt, struct vcpu *v),
         hvm_asid_flush_vcpu(v);
 
         cpu = read_atomic(&v->dirty_cpu);
-        if ( cpu != this_cpu && is_vcpu_dirty_cpu(cpu) )
+        if ( cpu != this_cpu && is_vcpu_dirty_cpu(cpu) && v->is_running )
             __cpumask_set_cpu(cpu, mask);
     }
 
@@ -729,7 +729,7 @@ static bool flush_tlb(bool (*flush_vcpu)(void *ctxt, struct vcpu *v),
      * not currently running will already be flushed when scheduled because of
      * the ASID tickle done in the loop above.
      */
-    on_selected_cpus(mask, dummy_flush, mask, 0);
+    on_selected_cpus(mask, dummy_flush, NULL, 0);
 
     return true;
 }