[Xen-devel] [PATCH v2] x86: do not enable global pages when virtualized on AMD hardware

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

When using global pages a full tlb flush can only be performed by
toggling the PGE bit in CR4, which is usually quite expensive in terms
of performance when running virtualized. This is specially relevant on
AMD hardware, which doesn't have the ability to do selective CR4
trapping, but can also be relevant on Intel if the underlying
hypervisor also traps accesses to the PGE CR4 bit.

In order to avoid this performance penalty, do not use global pages
when running virtualized on AMD hardware. A command line option
'global-pages' is provided in order to allow the user to select
whether global pages will be enabled for PV guests.

The above figures are from a PV shim running on AMD hardware with
32 vCPUs:

PGE enabled, x2APIC mode:

(XEN) Global lock flush_lock: addr=ffff82d0804b01c0, lockval=1adb1adb, not 
locked
(XEN)   lock:1841883(1375128998543), block:1658716(10193054890781)

Average lock time:   746588ns
Average block time: 6145147ns

PGE disabled, x2APIC mode:

(XEN) Global lock flush_lock: addr=ffff82d0804af1c0, lockval=a8bfa8bf, not 
locked
(XEN)   lock:2730175(657505389886), block:2039716(2963768247738)

Average lock time:   240829ns
Average block time: 1453029ns

As seen from the above figures the lock and block time of the flush
lock is reduced to approximately 1/3 of the original value.

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
Changes since v1:
 - Provide command line option to enable/disable PGE.
 - Only disable PGE on AMD hardware when virtualized.
 - Document the global-pages option.
---
 docs/misc/xen-command-line.pandoc | 13 +++++++++++++
 xen/arch/x86/pv/domain.c          |  9 ++++++++-
 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/docs/misc/xen-command-line.pandoc 
b/docs/misc/xen-command-line.pandoc
index d9495ef6b9..7be30f2766 100644
--- a/docs/misc/xen-command-line.pandoc
+++ b/docs/misc/xen-command-line.pandoc
@@ -1087,6 +1087,19 @@ value settable via Xen tools.
 
 Dom0 is using this value for sizing its maptrack table.
 
+### global-pages (x86)
+> `= <boolean>`
+
+> Default: `true` unless running virtualized on AMD hardware
+
+Set whether the PGE bit in CR4 will be enabled for PV guests. This controls the
+usage of global pages, and thus the need to perform tlb flushes by writing to
+CR4.
+
+Note it's disabled by default when running virtualized on AMD hardware since
+AMD SVM doesn't support selective trapping of CR4, so global pages are not
+enabled in order to reduce the overhead of tlb flushes.
+
 ### guest_loglvl
 > `= <level>[/<rate-limited level>]` where level is `none | error | warning | 
 > info | debug | all`
 
diff --git a/xen/arch/x86/pv/domain.c b/xen/arch/x86/pv/domain.c
index 4b6f48dea2..93fb823d63 100644
--- a/xen/arch/x86/pv/domain.c
+++ b/xen/arch/x86/pv/domain.c
@@ -118,11 +118,18 @@ unsigned long pv_fixup_guest_cr4(const struct vcpu *v, 
unsigned long cr4)
             (mmu_cr4_features & PV_CR4_GUEST_VISIBLE_MASK));
 }
 
+static int opt_global_pages = -1;
+boolean_runtime_param("global-pages", opt_global_pages);
+
 unsigned long pv_make_cr4(const struct vcpu *v)
 {
     const struct domain *d = v->domain;
     unsigned long cr4 = mmu_cr4_features &
         ~(X86_CR4_PCIDE | X86_CR4_PGE | X86_CR4_TSD);
+    bool pge = opt_global_pages == -1 ? (!cpu_has_hypervisor ||
+                                         boot_cpu_data.x86_vendor !=
+                                         X86_VENDOR_AMD)
+                                      : !!opt_global_pages;
 
     /*
      * PCIDE or PGE depends on the PCID/XPTI settings, but must not both be
@@ -130,7 +137,7 @@ unsigned long pv_make_cr4(const struct vcpu *v)
      */
     if ( d->arch.pv.pcid )
         cr4 |= X86_CR4_PCIDE;
-    else if ( !d->arch.pv.xpti )
+    else if ( !d->arch.pv.xpti && pge )
         cr4 |= X86_CR4_PGE;
 
     /*
-- 
2.24.0


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