Re: [XEN v6 2/3] xen/arm: arm64: Add emulation of Debug Data Transfer Registers

[Julien Grall <julien@xxxxxxx>]

Hi Ayan,

On 07/03/2024 12:39, Ayan Kumar Halder wrote:
> From: Michal Orzel <michal.orzel@xxxxxxx>
>
> Currently, if user enables HVC_DCC config option in Linux, it invokes access
> to debug data transfer registers (i.e. DBGDTRTX_EL0 on arm64, DBGDTRTXINT on
> arm32). As these registers are not emulated, Xen injects an undefined
> exception to the guest and Linux crashes.
>
> To prevent this crash, introduce a partial emulation of DBGDTR[TR]X_EL0
> (these registers share the same encoding) as RAZ/WI and MDCCSR_EL0 as TXfull.
>
> Refer ARM DDI 0487J.a ID042523, D19.3.8, DBGDTRTX_EL0
> "If TXfull is set to 1, set DTRRX and DTRTX to UNKNOWN".
>
> Thus, any OS is expected to read MDCCSR_EL0 and check for TXfull before
> using DBGDTRTX_EL0. Linux does it via hvc_dcc_init() ---> hvc_dcc_check(),
> and returns -ENODEV in case TXfull bit is still set after writing a test
> character. This way we prevent the guest from making use of HVC DCC as a
> console.
>
> Signed-off-by: Michal Orzel <michal.orzel@xxxxxxx>
> Signed-off-by: Ayan Kumar Halder <ayan.kumar.halder@xxxxxxx>
Acked-by: Julien Grall <jgrall@xxxxxxxxxx>

Cheers,

--
Julien Grall