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

[Timur Tabi]

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.
>
> > >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.
>
> > >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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