[Xen-changelog] [xen staging] x86/hvm: allow ASID flush when v != current

[patchbot@xxxxxxx]

commit a798bac54fe87a7d69fb7be7f125b8c97f767ff6
Author:     Roger Pau Monné <roger.pau@xxxxxxxxxx>
AuthorDate: Fri Mar 6 10:18:13 2020 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Fri Mar 6 10:18:13 2020 +0100

    x86/hvm: allow ASID flush when v != current
    
    Current implementation of hvm_asid_flush_vcpu is not safe to use
    unless the target vCPU is either paused or the currently running one,
    as it modifies the generation without any locking.
    
    Fix this by using atomic operations when accessing the generation
    field, both in hvm_asid_flush_vcpu_asid and other ASID functions. This
    allows to safely flush the current ASID generation. Note that for the
    flush to take effect if the vCPU is currently running a vmexit is
    required.
    
    Compilers will normally do such writes and reads as a single
    instruction, so the usage of atomic operations is mostly used as a
    safety measure.
    
    Note the same could be achieved by introducing an extra field to
    hvm_vcpu_asid that signals hvm_asid_handle_vmenter the need to call
    hvm_asid_flush_vcpu on the given vCPU before vmentry, this however
    seems unnecessary as hvm_asid_flush_vcpu itself only sets two vCPU
    fields to 0, so there's no need to delay this to the vmentry ASID
    helper.
    
    This is not a bugfix as no callers that would violate the assumptions
    listed in the first paragraph have been found, but a preparatory
    change in order to allow remote flushing of HVM vCPUs.
    
    Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    Reviewed-by: Wei Liu <wl@xxxxxxx>
    Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/hvm/asid.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/xen/arch/x86/hvm/asid.c b/xen/arch/x86/hvm/asid.c
index d61d7fc33c..1fd2770a3a 100644
--- a/xen/arch/x86/hvm/asid.c
+++ b/xen/arch/x86/hvm/asid.c
@@ -83,7 +83,7 @@ void hvm_asid_init(int nasids)
 
 void hvm_asid_flush_vcpu_asid(struct hvm_vcpu_asid *asid)
 {
-    asid->generation = 0;
+    write_atomic(&asid->generation, 0);
 }
 
 void hvm_asid_flush_vcpu(struct vcpu *v)
@@ -121,7 +121,7 @@ bool_t hvm_asid_handle_vmenter(struct hvm_vcpu_asid *asid)
         goto disabled;
 
     /* Test if VCPU has valid ASID. */
-    if ( asid->generation == data->core_asid_generation )
+    if ( read_atomic(&asid->generation) == data->core_asid_generation )
         return 0;
 
     /* If there are no free ASIDs, need to go to a new generation */
@@ -135,7 +135,7 @@ bool_t hvm_asid_handle_vmenter(struct hvm_vcpu_asid *asid)
 
     /* Now guaranteed to be a free ASID. */
     asid->asid = data->next_asid++;
-    asid->generation = data->core_asid_generation;
+    write_atomic(&asid->generation, data->core_asid_generation);
 
     /*
      * When we assign ASID 1, flush all TLB entries as we are starting a new
--
generated by git-patchbot for /home/xen/git/xen.git#staging

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