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Re: [Xen-devel] [PATCH for-4.5 v6 04/16] xen: Add vmware_port support

On 09/23/14 13:16, Boris Ostrovsky wrote:
On 09/20/2014 02:07 PM, Don Slutz wrote:
@@ -2064,6 +2065,42 @@ svm_vmexit_do_vmsave(struct vmcb_struct *vmcb,
  +static void svm_vmexit_gp_intercept(struct cpu_user_regs *regs,
+                                    struct vcpu *v)
+    struct vmcb_struct *vmcb = v->arch.hvm_svm.vmcb;
+    /*
+     * Just use 15 for the instruction length; vmport_gp_check will
+     * adjust it.  This is because
+     * __get_instruction_length_from_list() has issues, and may
+     * require a double read of the instruction bytes.  At some
+     * point a new routine could be added that is based on the code
+     * in vmport_gp_check with extensions to make it more general.
+     * Since that routine is the only user of this code this can be
+     * done later.
+     */
+    unsigned long inst_len = 15;

Can you add a comment describing why you chose 15?

Also, saying that __get_instruction_length_from_list() has issues I think requires a bit more details (e.g. that when called from #GP handler NRIP is not available, or that NRIP may not be available at all on a particular HW, leading to the need read the instruction twice --- once in __get_instruction_length_from_list() and then again in vmport_gp_check(). Which is bad because memory may change between the reads. Or something like that.).

Added more on the commit message about this.

   -Don Slutz


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