xen/domctl: Drop vcpu_alloc_lock

Since its introduction in c/s 8cbb5278e "x86/AMD: Add support for AMD's OSVW
feature in guests", the OSVW data has been corrected to be per-domain rather
than per-vcpu, and is initialised during XEN_DOMCTL_createdomain.

Furthermore, because XENPF_microcode_update uses hypercall continuations to
move between CPUs, it drops the vcpu_alloc_lock mid update, meaning that it
didn't provided the interlock guarantee that the OSVW patch was looking for in
the first place.

This interlock serves no purpose, so take the opportunity to drop it and
remove a global spinlock from the hypervisor.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>

xen/arch/x86/platform_hypercall.c

@@ -280,24 +280,9 @@ ret_t do_platform_op(XEN_GUEST_HANDLE_PARAM(xen_platform_op_t) u_xenpf_op)
 
         guest_from_compat_handle(data, op->u.microcode.data);
 
-        /*
-         * alloc_vcpu() will access data which is modified during
-         * microcode update
-         */
-        while ( !spin_trylock(&vcpu_alloc_lock) )
-        {
-            if ( hypercall_preempt_check() )
-            {
-                ret = hypercall_create_continuation(
-                    __HYPERVISOR_platform_op, "h", u_xenpf_op);
-                goto out;
-            }
-        }
-
         ret = microcode_update(
                 guest_handle_to_param(data, const_void),
                 op->u.microcode.length);
-        spin_unlock(&vcpu_alloc_lock);
     }
     break;
 

xen/common/domctl.c

@@ -34,7 +34,6 @@
 #include <xsm/xsm.h>
 
 static DEFINE_SPINLOCK(domctl_lock);
-DEFINE_SPINLOCK(vcpu_alloc_lock);
 
 static int bitmap_to_xenctl_bitmap(struct xenctl_bitmap *xenctl_bitmap,
                                    const unsigned long *bitmap,
@@ -560,20 +559,6 @@ long do_domctl(XEN_GUEST_HANDLE_PARAM(xen_domctl_t) u_domctl)
         /* Needed, for example, to ensure writable p.t. state is synced. */
         domain_pause(d);
 
-        /*
-         * Certain operations (e.g. CPU microcode updates) modify data which is
-         * used during VCPU allocation/initialization
-         */
-        while ( !spin_trylock(&vcpu_alloc_lock) )
-        {
-            if ( hypercall_preempt_check() )
-            {
-                ret =  hypercall_create_continuation(
-                    __HYPERVISOR_domctl, "h", u_domctl);
-                goto maxvcpu_out_novcpulock;
-            }
-        }
-
         ret = -ENOMEM;
         online = cpupool_domain_cpumask(d);
 
@@ -593,9 +578,6 @@ long do_domctl(XEN_GUEST_HANDLE_PARAM(xen_domctl_t) u_domctl)
         ret = 0;
 
     maxvcpu_out:
-        spin_unlock(&vcpu_alloc_lock);
-
-    maxvcpu_out_novcpulock:
         domain_unpause(d);
         break;
     }

xen/include/xen/domain.h

@@ -87,7 +87,6 @@ void arch_dump_domain_info(struct domain *d);
 
 int arch_vcpu_reset(struct vcpu *);
 
-extern spinlock_t vcpu_alloc_lock;
 bool_t domctl_lock_acquire(void);
 void domctl_lock_release(void);