Re: [Xen-devel] [PATCH] x86/debug: Avoid crashing in early boot because of debugger_trap_entry()

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 03.08.16 at 19:08, <andrew.cooper3@xxxxxxxxxx> wrote:
> debugger_trap_entry() is not safe to use during early boot, as it follows
> current before it is necesserily safe to do so.  Futhermore it does this
> unconditionally, despite most callsites turning into no-ops because of the
> vector test.
> 
> Inline debugger_trap_entry() into the two callers where doesn't become a
> no-op,

Implied from that you delete all other invocations. While from a
(current) functionality pov that's no functional change, to me these
invocations have always served as an indication that some action
_could_ be taken there as well. I'm not sure it's a good idea to
remove that.

> @@ -3814,6 +3825,13 @@ void do_debug(struct cpu_user_regs *regs)
>      /* Save debug status register where guest OS can peek at it */
>      v->arch.debugreg[6] = read_debugreg(6);
>  
> +    if ( guest_kernel_mode(v, regs) && v->domain->debugger_attached )
> +    {
> +        v->arch.gdbsx_vcpu_event = TRAP_debug;

Note how in the original code this write didn't happen. I can't tell why,
but it's a behavioral change which certainly would need explaining and
blessing by the gdbsx maintainer.

Jan


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