[PATCH 3/3] x86/xstate: re-size save area when CPUID policy changes

[Jan Beulich <jbeulich@xxxxxxxx>]

vCPU-s get maximum size areas allocated initially. Hidden (and in
particular default-off) features may allow for a smaller size area to
suffice.

Suggested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
Seeing that both vcpu_init_fpu() and cpuid_policy_updated() get called
from arch_vcpu_create(), I'm not sure we really need this two-stage
approach - the slightly longer period of time during which
v->arch.xsave_area would remain NULL doesn't look all that problematic.
But since xstate_alloc_save_area() gets called for idle vCPU-s, it has
to stay anyway in some form, so the extra code churn may not be worth
it.

--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -294,7 +294,21 @@ void update_guest_memory_policy(struct v
     }
 }
 
-void domain_cpu_policy_changed(struct domain *d)
+/*
+ * Called during vcpu construction, and each time the toolstack changes the
+ * CPUID configuration for the domain.
+ */
+static int __must_check cpuid_policy_updated(struct vcpu *v)
+{
+    int rc = xstate_update_save_area(v);
+
+    if ( !rc && is_hvm_vcpu(v) )
+        hvm_cpuid_policy_changed(v);
+
+    return rc;
+}
+
+int domain_cpu_policy_changed(struct domain *d)
 {
     const struct cpuid_policy *p = d->arch.cpuid;
     struct vcpu *v;
@@ -452,13 +466,18 @@ void domain_cpu_policy_changed(struct do
 
     for_each_vcpu ( d, v )
     {
-        cpuid_policy_updated(v);
+        int rc = cpuid_policy_updated(v);
+
+        if ( rc )
+            return rc;
 
         /* If PMU version is zero then the guest doesn't have VPMU */
         if ( boot_cpu_data.x86_vendor == X86_VENDOR_INTEL &&
              p->basic.pmu_version == 0 )
             vpmu_destroy(v);
     }
+
+    return 0;
 }
 
 #ifndef CONFIG_BIGMEM
@@ -597,7 +616,7 @@ int arch_vcpu_create(struct vcpu *v)
     {
         vpmu_initialise(v);
 
-        cpuid_policy_updated(v);
+        rc = cpuid_policy_updated(v);
     }
 
     return rc;
@@ -841,9 +860,9 @@ int arch_domain_create(struct domain *d,
      */
     d->arch.x87_fip_width = cpu_has_fpu_sel ? 0 : 8;
 
-    domain_cpu_policy_changed(d);
-
-    return 0;
+    rc = domain_cpu_policy_changed(d);
+    if ( !rc )
+        return 0;
 
  fail:
     d->is_dying = DOMDYING_dead;
@@ -2434,16 +2453,6 @@ int domain_relinquish_resources(struct d
     return 0;
 }
 
-/*
- * Called during vcpu construction, and each time the toolstack changes the
- * CPUID configuration for the domain.
- */
-void cpuid_policy_updated(struct vcpu *v)
-{
-    if ( is_hvm_vcpu(v) )
-        hvm_cpuid_policy_changed(v);
-}
-
 void arch_dump_domain_info(struct domain *d)
 {
     paging_dump_domain_info(d);
--- a/xen/arch/x86/domctl.c
+++ b/xen/arch/x86/domctl.c
@@ -91,7 +91,7 @@ static int update_domain_cpu_policy(stru
     recalculate_cpuid_policy(d);
 
     /* Recalculate relevant dom/vcpu state now the policy has changed. */
-    domain_cpu_policy_changed(d);
+    ret = domain_cpu_policy_changed(d);
 
  out:
     /* Free whichever cpuid/msr structs are not installed in struct domain. */
--- a/xen/arch/x86/xstate.c
+++ b/xen/arch/x86/xstate.c
@@ -541,6 +541,41 @@ int xstate_alloc_save_area(struct vcpu *
 
     return 0;
 }
+
+int xstate_update_save_area(struct vcpu *v)
+{
+    unsigned int i, size, old;
+    struct xsave_struct *save_area;
+    uint64_t xcr0_max = cpuid_policy_xcr0_max(v->domain->arch.cpuid);
+
+    ASSERT(!is_idle_vcpu(v));
+
+    if ( !cpu_has_xsave )
+        return 0;
+
+    if ( v->arch.xcr0_accum & ~xcr0_max )
+        return -EBUSY;
+
+    for ( size = old = XSTATE_AREA_MIN_SIZE, i = 2; i < xstate_features; ++i )
+    {
+        if ( xcr0_max & (1ull << i) )
+            size = max(size, xstate_offsets[i] + xstate_sizes[i]);
+        if ( v->arch.xcr0_accum & (1ull << i) )
+            old = max(old, xstate_offsets[i] + xstate_sizes[i]);
+    }
+
+    save_area = _xvrealloc(v->arch.xsave_area, size, __alignof(*save_area));
+    if ( !save_area )
+        return -ENOMEM;
+
+    ASSERT(old <= size);
+    memset((void *)save_area + old, 0, size - old);
+
+    v->arch.xsave_area = save_area;
+    v->arch.fpu_ctxt = &v->arch.xsave_area->fpu_sse;
+
+    return 0;
+}
 
 void xstate_free_save_area(struct vcpu *v)
 {
--- a/xen/include/asm-x86/domain.h
+++ b/xen/include/asm-x86/domain.h
@@ -78,8 +78,6 @@ void toggle_guest_mode(struct vcpu *);
 /* x86/64: toggle guest page tables between kernel and user modes. */
 void toggle_guest_pt(struct vcpu *);
 
-void cpuid_policy_updated(struct vcpu *v);
-
 /*
  * Initialise a hypercall-transfer page. The given pointer must be mapped
  * in Xen virtual address space (accesses are not validated or checked).
@@ -667,7 +665,7 @@ struct guest_memory_policy
 void update_guest_memory_policy(struct vcpu *v,
                                 struct guest_memory_policy *policy);
 
-void domain_cpu_policy_changed(struct domain *d);
+int __must_check domain_cpu_policy_changed(struct domain *d);
 
 bool update_runstate_area(struct vcpu *);
 bool update_secondary_system_time(struct vcpu *,
--- a/xen/include/asm-x86/xstate.h
+++ b/xen/include/asm-x86/xstate.h
@@ -106,6 +106,7 @@ void compress_xsave_states(struct vcpu *
 /* extended state init and cleanup functions */
 void xstate_free_save_area(struct vcpu *v);
 int xstate_alloc_save_area(struct vcpu *v);
+int xstate_update_save_area(struct vcpu *v);
 void xstate_init(struct cpuinfo_x86 *c);
 unsigned int xstate_ctxt_size(u64 xcr0);