Re: [PATCH 2/3] x86/pv: Don't clobber NT on return-to-guest

[Jan Beulich <jbeulich@xxxxxxxx>]

On 23.09.2020 12:18, Andrew Cooper wrote:
> A 64bit IRET can restore NT - the faulting case is when NT is set in the live
> flags.  This change had an unintended consequence of causing the NT flag to
> spontaneously disappear from guest context whenever a interrupt/exception
> occurred.
> 
> In combination with a SYSENTER which sets both TF and NT, Xen's handling of
> the #DB exceptions clears NT before it is even recorded suitably in the guest
> kernel's view of what userspace was doing.
> 
> Reported-by: Andy Lutomirski <luto@xxxxxxxxxx>
> Fixes: 0e47f92b0 ("x86: force EFLAGS.IF on when exiting to PV guests")
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>