x86: do not enable global pages when virtualized on AMD or Hygon hardware

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 or Hygon hardware, which doesn't have the ability to do selective
CR4 trapping, but can also be relevant on e.g. 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 or Hygon 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.

Note that XEN_MINIMAL_CR4 and mmu_cr4_features are not modified, and
thus global pages are left enabled for the hypervisor. This is not an
issue because the code to switch the control registers (cr3 and cr4)
already takes into account such situation and performs the necessary
flushes. The same already happens when using XPTI or PCIDE, as the
guest cr4 doesn't have global pages enabled in that case either.

Also note that the suspend and resume code is correct in writing
mmu_cr4_features into cr4 on resume, since that's the cr4 used by the
idle vCPU which is the context used by the suspend and resume routine.

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

diff --git a/docs/misc/xen-command-line.pandoc b/docs/misc/xen-command-line.pandoc
index 891d2d439fefa199413b753e8d7c261042ac9070..7a1be84ca9c1fb4f47576dc58d11b1e4e51a8c96 100644 (file)
--- 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 or Hygon 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 or Hygon 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 4b6f48dea23d2b7bd6490c2aa7973b88cb6d6b05..e6e1c51548bf14252c92726b587a3fe9d9487c4e 100644 (file)
--- a/xen/arch/x86/pv/domain.c
+++ b/xen/arch/x86/pv/domain.c
@@ -118,6 +118,20 @@ unsigned long pv_fixup_guest_cr4(const struct vcpu *v, unsigned long cr4)
             (mmu_cr4_features & PV_CR4_GUEST_VISIBLE_MASK));
 }
 
+static int8_t __read_mostly opt_global_pages = -1;
+boolean_runtime_param("global-pages", opt_global_pages);
+
+static int __init pge_init(void)
+{
+    if ( opt_global_pages == -1 )
+        opt_global_pages = !cpu_has_hypervisor ||
+                           (boot_cpu_data.x86_vendor &
+                            (X86_VENDOR_AMD | X86_VENDOR_HYGON));
+
+    return 0;
+}
+__initcall(pge_init);
+
 unsigned long pv_make_cr4(const struct vcpu *v)
 {
     const struct domain *d = v->domain;
@@ -130,7 +144,7 @@ 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 && opt_global_pages )
         cr4 |= X86_CR4_PGE;
 
     /*