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

["George Dunlap " <dunlapg@xxxxxxxxx>]

Good catch.  Do you have a simple test to verify that the new patch
works properly?
-George

On 8/17/06, Travis Betak <travis.betak@xxxxxxx> wrote:
> 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;
>
> ---
>
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