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

From: Julien Grall <julien@xen.org>
To: Ayan Kumar Halder <ayan.kumar.halder@amd.com>,
	xen-devel@lists.xenproject.org
Cc: sstabellini@kernel.org, stefano.stabellini@amd.com,
	Volodymyr_Babchuk@epam.com, bertrand.marquis@arm.com,
	michal.orzel@amd.com
Subject: Re: [XEN v6 2/3] xen/arm: arm64: Add emulation of Debug Data Transfer Registers
Date: Tue, 12 Mar 2024 22:27:53 +0000	[thread overview]
Message-ID: <ede601d8-804b-46d5-955f-eea895b269ba@xen.org> (raw)
In-Reply-To: <20240307123943.1991755-3-ayan.kumar.halder@amd.com>

Hi Ayan,

On 07/03/2024 12:39, Ayan Kumar Halder wrote:
> From: Michal Orzel <michal.orzel@amd.com>
> 
> 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@amd.com>
> Signed-off-by: Ayan Kumar Halder <ayan.kumar.halder@amd.com>

Acked-by: Julien Grall <jgrall@amazon.com>

Cheers,

-- 
Julien Grall