linux-kernel - Re: [PATCH] ARM64: TTY: hvc_dcc: Add support for ARM64 dcc

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Date:	Wed, 24 Jun 2015 15:11:24 -0500
From:	Timur Tabi <timur@...eaurora.org>
To:	Will Deacon <will.deacon@....com>
CC:	Catalin Marinas <Catalin.Marinas@....com>,
	"abhimany@...eaurora.org" <abhimany@...eaurora.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	sboyd@...eaurora.org
Subject: Re: [PATCH] ARM64: TTY: hvc_dcc: Add support for ARM64 dcc

On 06/22/2015 08:12 AM, Will Deacon wrote:
> I still think we should be disabling userspace access to the DCC if the
> kernel is using it as its console.

I still need help with this.  I know you said a year ago that 
MDSCR_EL1.TDCC needs to be set to disable userspace access.  Where and 
how should I do this?  I can do this:

static int __init hvc_dcc_console_init(void)
{
#ifdef CONFIG_ARM64
	u32 val;

	asm("msr mdscr_el1, %0	"
	"orr %0, %0, #4096	" /* TDCC */
	"msr %0, mdscr_el1	"
	: "=r" (val));
#endif

But this seems clunky.

I am concerned about KVM, though.  There appears to be code in KVM in 
hyp.s and sys_regs.c that touches and/or emulates MDSCR_EL1.

On a side note, it does not appear that ARM32 blocks userspace DCC.  I 
don't see where DBGDSCR.UDCCdis is set.

-- 
Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the
Code Aurora Forum, a Linux Foundation Collaborative Project.
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