x86/hvm/viridian: enable APIC assist enlightenment

This patch adds code to enable the APIC assist enlightenment which,
under certain conditions, means that the guest can avoid an EOI of
the local APIC and thereby avoid a VMEXIT.

Use of the enlightenment by the hypervisor is under control of the
toolstack, and is added to the default set.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>

docs/man/xl.cfg.pod.5

@@ -1568,10 +1568,20 @@ This set incorporates use of hypercalls for remote TLB flushing.
 This enlightenment may improve performance of Windows guests running
 on hosts with higher levels of (physical) CPU contention.
 
+=item B<apic_assist>
+
+This set incorporates use of the APIC assist page to avoid EOI of
+the local APIC.
+This enlightenment may improve performance of guests that make use of
+per-vcpu event channel upcall vectors.
+Note that this enlightenment will have no effect if the guest is
+using APICv posted interrupts.
+
 =item B<defaults>
 
 This is a special value that enables the default set of groups, which
-is currently the B<base>, B<freq> and B<time_ref_count> groups.
+is currently the B<base>, B<freq>, B<time_ref_count> and B<apic_assist>
+groups.
 
 =item B<all>
 

tools/libxl/libxl.h

@@ -223,6 +223,12 @@
  */
 #define LIBXL_HAVE_SOFT_RESET 1
 
+/*
+ * LIBXL_HAVE_APIC_ASSIST indicates that the 'apic_assist' value
+ * is present in the viridian enlightenment enumeration.
+ */
+#define LIBXL_HAVE_APIC_ASSIST 1
+
 /*
  * libxl ABI compatibility
  *

tools/libxl/libxl_dom.c

@@ -211,6 +211,7 @@ static int hvm_set_viridian_features(libxl__gc *gc, uint32_t domid,
         libxl_bitmap_set(&enlightenments, LIBXL_VIRIDIAN_ENLIGHTENMENT_BASE);
         libxl_bitmap_set(&enlightenments, LIBXL_VIRIDIAN_ENLIGHTENMENT_FREQ);
         libxl_bitmap_set(&enlightenments, LIBXL_VIRIDIAN_ENLIGHTENMENT_TIME_REF_COUNT);
+        libxl_bitmap_set(&enlightenments, LIBXL_VIRIDIAN_ENLIGHTENMENT_APIC_ASSIST);
     }
 
     libxl_for_each_set_bit(v, info->u.hvm.viridian_enable) {
@@ -253,6 +254,9 @@ static int hvm_set_viridian_features(libxl__gc *gc, uint32_t domid,
     if (libxl_bitmap_test(&enlightenments, LIBXL_VIRIDIAN_ENLIGHTENMENT_HCALL_REMOTE_TLB_FLUSH))
         mask |= HVMPV_hcall_remote_tlb_flush;
 
+    if (libxl_bitmap_test(&enlightenments, LIBXL_VIRIDIAN_ENLIGHTENMENT_APIC_ASSIST))
+        mask |= HVMPV_apic_assist;
+
     if (mask != 0 &&
         xc_hvm_param_set(CTX->xch,
                          domid,

tools/libxl/libxl_types.idl

@@ -221,6 +221,7 @@ libxl_viridian_enlightenment = Enumeration("viridian_enlightenment", [
     (2, "time_ref_count"),
     (3, "reference_tsc"),
     (4, "hcall_remote_tlb_flush"),
+    (5, "apic_assist"),
     ])
 
 libxl_hdtype = Enumeration("hdtype", [

xen/arch/x86/hvm/viridian.c

@@ -227,11 +227,6 @@ static void initialize_apic_assist(struct vcpu *v)
     void *va;
 
     /*
-     * We don't yet make use of the APIC assist page but by setting
-     * the CPUID3A_MSR_APIC_ACCESS bit in CPUID leaf 40000003 we are duty
-     * bound to support the MSR. We therefore do just enough to keep windows
-     * happy.
-     *
      * See section 13.3.4.1 of the specification for details of this
      * enlightenment.
      */
@@ -254,7 +249,15 @@ static void initialize_apic_assist(struct vcpu *v)
 
     *(uint32_t *)va = 0;
 
-    v->arch.hvm_vcpu.viridian.apic_assist.va = va;
+    if ( viridian_feature_mask(v->domain) & HVMPV_apic_assist )
+    {
+        v->arch.hvm_vcpu.viridian.apic_assist.va = va;
+        v->arch.hvm_vcpu.viridian.apic_assist.vector = -1;
+        return;
+    }
+
+    unmap_domain_page_global(va);
+    put_page_and_type(page);
     return;
 
  fail:
@@ -278,6 +281,53 @@ static void teardown_apic_assist(struct vcpu *v)
     put_page_and_type(page);
 }
 
+void viridian_start_apic_assist(struct vcpu *v, int vector)
+{
+    uint32_t *va = v->arch.hvm_vcpu.viridian.apic_assist.va;
+
+    if ( !va )
+        return;
+
+    /*
+     * If there is already an assist pending then something has gone
+     * wrong and the VM will most likely hang so force a crash now
+     * to make the problem clear.
+     */
+    if ( v->arch.hvm_vcpu.viridian.apic_assist.vector >= 0 )
+        domain_crash(v->domain);
+
+    v->arch.hvm_vcpu.viridian.apic_assist.vector = vector;
+    *va |= 1u;
+}
+
+int viridian_complete_apic_assist(struct vcpu *v)
+{
+    uint32_t *va = v->arch.hvm_vcpu.viridian.apic_assist.va;
+    int vector;
+
+    if ( !va )
+        return -1;
+
+    if ( *va & 1u )
+        return -1; /* Interrupt not yet processed by the guest. */
+
+    vector = v->arch.hvm_vcpu.viridian.apic_assist.vector;
+    v->arch.hvm_vcpu.viridian.apic_assist.vector = -1;
+
+    return vector;
+}
+
+void viridian_abort_apic_assist(struct vcpu *v)
+{
+    uint32_t *va = v->arch.hvm_vcpu.viridian.apic_assist.va;
+
+    if ( !va )
+        return;
+
+    *va &= ~1u;
+    v->arch.hvm_vcpu.viridian.apic_assist.vector = -1;
+}
+
 static void update_reference_tsc(struct domain *d, bool_t initialize)
 {
     unsigned long gmfn = d->arch.hvm_domain.viridian.reference_tsc.fields.pfn;

xen/arch/x86/hvm/vlapic.c

@@ -38,6 +38,7 @@
 #include <asm/hvm/support.h>
 #include <asm/hvm/vmx/vmx.h>
 #include <asm/hvm/nestedhvm.h>
+#include <asm/hvm/viridian.h>
 #include <public/hvm/ioreq.h>
 #include <public/hvm/params.h>
 
@@ -95,6 +96,18 @@ static int vlapic_find_highest_vector(const void *bitmap)
     return (fls(word[word_offset*4]) - 1) + (word_offset * 32);
 }
 
+static int vlapic_find_lowest_vector(const void *bitmap)
+{
+    const uint32_t *word = bitmap;
+    unsigned int word_offset;
+
+    /* Work forwards through the bitmap (first 32-bit word in every four). */
+    for ( word_offset = 0; word_offset < NR_VECTORS / 32; word_offset++)
+        if ( word[word_offset * 4] )
+            return (ffs(word[word_offset * 4]) - 1) + (word_offset * 32);
+
+    return -1;
+}
 
 /*
  * IRR-specific bitmap update & search routines.
@@ -1157,7 +1170,7 @@ int vlapic_virtual_intr_delivery_enabled(void)
 int vlapic_has_pending_irq(struct vcpu *v)
 {
     struct vlapic *vlapic = vcpu_vlapic(v);
-    int irr, isr;
+    int irr, vector, isr;
 
     if ( !vlapic_enabled(vlapic) )
         return -1;
@@ -1170,10 +1183,27 @@ int vlapic_has_pending_irq(struct vcpu *v)
          !nestedhvm_vcpu_in_guestmode(v) )
         return irr;
 
+    /*
+     * If APIC assist was used then there may have been no EOI so
+     * we need to clear the requisite bit from the ISR here, before
+     * comparing with the IRR.
+     */
+    vector = viridian_complete_apic_assist(v);
+    if ( vector != -1 )
+        vlapic_clear_vector(vector, &vlapic->regs->data[APIC_ISR]);
+
     isr = vlapic_find_highest_isr(vlapic);
     isr = (isr != -1) ? isr : 0;
     if ( (isr & 0xf0) >= (irr & 0xf0) )
+    {
+        /*
+         * There's already a higher priority vector pending so
+         * we need to abort any previous APIC assist to ensure there
+         * is an EOI.
+         */
+        viridian_abort_apic_assist(v);
         return -1;
+    }
 
     return irr;
 }
@@ -1181,13 +1211,31 @@ int vlapic_has_pending_irq(struct vcpu *v)
 int vlapic_ack_pending_irq(struct vcpu *v, int vector, bool_t force_ack)
 {
     struct vlapic *vlapic = vcpu_vlapic(v);
+    int isr;
 
-    if ( force_ack || !vlapic_virtual_intr_delivery_enabled() )
-    {
-        vlapic_set_vector(vector, &vlapic->regs->data[APIC_ISR]);
-        vlapic_clear_irr(vector, vlapic);
-    }
+    if ( !force_ack &&
+         vlapic_virtual_intr_delivery_enabled() )
+        return 1;
 
+    /* If there's no chance of using APIC assist then bail now. */
+    if ( !has_viridian_apic_assist(v->domain) ||
+         vlapic_test_vector(vector, &vlapic->regs->data[APIC_TMR]) )
+        goto done;
+
+    isr = vlapic_find_lowest_vector(&vlapic->regs->data[APIC_ISR]);
+    if ( isr >= 0 && isr < vector )
+        goto done;
+
+    /*
+     * This vector is edge triggered and there are no lower priority
+     * vectors pending, so we can use APIC assist to avoid exiting
+     * for EOI.
+     */
+    viridian_start_apic_assist(v, vector);
+
+ done:
+    vlapic_set_vector(vector, &vlapic->regs->data[APIC_ISR]);
+    vlapic_clear_irr(vector, vlapic);
     return 1;
 }
 

xen/include/asm-x86/hvm/hvm.h

@@ -402,6 +402,9 @@ static inline unsigned long hvm_get_shadow_gs_base(struct vcpu *v)
 #define has_viridian_time_ref_count(d) \
     (is_viridian_domain(d) && (viridian_feature_mask(d) & HVMPV_time_ref_count))
 
+#define has_viridian_apic_assist(d) \
+    (is_viridian_domain(d) && (viridian_feature_mask(d) & HVMPV_apic_assist))
+
 void hvm_hypervisor_cpuid_leaf(uint32_t sub_idx,
                                uint32_t *eax, uint32_t *ebx,
                                uint32_t *ecx, uint32_t *edx);

xen/include/asm-x86/hvm/viridian.h

@@ -24,6 +24,7 @@ struct viridian_vcpu
     struct {
         union viridian_apic_assist msr;
         void *va;
+        int vector;
     } apic_assist;
 };
 
@@ -122,6 +123,10 @@ void viridian_time_ref_count_thaw(struct domain *d);
 
 void viridian_vcpu_deinit(struct vcpu *v);
 
+void viridian_start_apic_assist(struct vcpu *v, int vector);
+int viridian_complete_apic_assist(struct vcpu *v);
+void viridian_abort_apic_assist(struct vcpu *v);
+
 #endif /* __ASM_X86_HVM_VIRIDIAN_H__ */
 
 /*

xen/include/public/hvm/params.h

@@ -115,12 +115,17 @@
 #define _HVMPV_hcall_remote_tlb_flush 4
 #define HVMPV_hcall_remote_tlb_flush (1 << _HVMPV_hcall_remote_tlb_flush)
 
+/* Use APIC assist */
+#define _HVMPV_apic_assist 5
+#define HVMPV_apic_assist (1 << _HVMPV_apic_assist)
+
 #define HVMPV_feature_mask \
         (HVMPV_base_freq | \
          HVMPV_no_freq | \
          HVMPV_time_ref_count | \
          HVMPV_reference_tsc | \
-         HVMPV_hcall_remote_tlb_flush)
+         HVMPV_hcall_remote_tlb_flush | \
+         HVMPV_apic_assist)
 
 #endif