[Xen-devel] [HVM] Debug register access question

["Travis Betak" <travis.betak@xxxxxxx>]

Hello all,

I'm adding the lazy save/restore of hardware debug registers to the SVM 
part of HVM and I'm slightly confused about something done in the VMX 
counterpart.
The intercept for debug registers is enabled until the first access.  The 
hypervisor takes note that the guests debug registers are now dirty and 
disables the debug register intercept allowing the guest free access to 
the registers.
My question is this.  On the debug register access, why is the EIP 
updated?  I don't understand some of the finer details of VT but by 
updating the EIP, doesn't this initial debug register access get lost? 
I've included the two snippets of code in question below and a patch to 
remove EIP update if it is wrong.
  --travis

---

static void vmx_dr_access(unsigned long exit_qualification,
                          struct cpu_user_regs *regs)
{
    struct vcpu *v = current;

    v->arch.hvm_vcpu.flag_dr_dirty = 1;

    /* We could probably be smarter about this */
    __restore_debug_registers(v);

    /* Allow guest direct access to DR registers */
    v->arch.hvm_vcpu.u.vmx.exec_control &= ~CPU_BASED_MOV_DR_EXITING;
    __vmwrite(CPU_BASED_VM_EXEC_CONTROL,
              v->arch.hvm_vcpu.u.vmx.exec_control);
}

[snip ...]

    case EXIT_REASON_DR_ACCESS:
        __vmread(EXIT_QUALIFICATION, &exit_qualification);
        vmx_dr_access(exit_qualification, &regs);
        __get_instruction_length(inst_len);
        __update_guest_eip(inst_len);
        break;

---

dont-update-eip-on-dr-access.patch
Description: Text document

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