[PATCH] ARM64: TTY: hvc_dcc: Add support for ARM64 dcc

Will Deacon wrote:

Well, it's up to you to figure out the details, but I'd start by adding
some static inlines to the arch-specific header files for enabling/disabling
userspace access.

 From there, I think I'd get the architecture init code to reset the thing
to "disabled" (so it's disabled regardless of whether we build the hvc_dcc
driver) and then if you wanted to go all-out, we could have a sysfs entry
provided by the driver to toggle it on and off.

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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.

Yeah, that's super ugly.

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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.

That's a bug imo.

So wouldn't it be more appropriate to have a separate patch that handles 
disabling of user-space DCC for ARM32 and ARM64?  All I really want to 
do at this point is provide basic DCC support for ARM64, just like we 
have for ARM32.

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