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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>
CC: Andy Lutomirski <luto@xxxxxxxxxx>
---
 xen/arch/x86/x86_64/compat/entry.S | 2 +-
 xen/arch/x86/x86_64/entry.S        | 4 ++--
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/xen/arch/x86/x86_64/compat/entry.S 
b/xen/arch/x86/x86_64/compat/entry.S
index 73619f57ca..3b2136b272 100644
--- a/xen/arch/x86/x86_64/compat/entry.S
+++ b/xen/arch/x86/x86_64/compat/entry.S
@@ -119,7 +119,7 @@ compat_process_trap:
 /* %rbx: struct vcpu, interrupts disabled */
 ENTRY(compat_restore_all_guest)
         ASSERT_INTERRUPTS_DISABLED
-        mov   $~(X86_EFLAGS_IOPL|X86_EFLAGS_NT|X86_EFLAGS_VM),%r11d
+        mov   $~(X86_EFLAGS_IOPL | X86_EFLAGS_VM), %r11d
         and   UREGS_eflags(%rsp),%r11d
 
 .macro alt_cr4_pv32
diff --git a/xen/arch/x86/x86_64/entry.S b/xen/arch/x86/x86_64/entry.S
index 44a110b9c8..000eb9722b 100644
--- a/xen/arch/x86/x86_64/entry.S
+++ b/xen/arch/x86/x86_64/entry.S
@@ -182,7 +182,7 @@ restore_all_guest:
         jz    iret_exit_to_guest
 
         movq  24(%rsp),%r11           # RFLAGS
-        andq  $~(X86_EFLAGS_IOPL|X86_EFLAGS_NT|X86_EFLAGS_VM),%r11
+        andq  $~(X86_EFLAGS_IOPL | X86_EFLAGS_VM), %r11
         orq   $X86_EFLAGS_IF,%r11
 
         /* Don't use SYSRET path if the return address is not canonical. */
@@ -213,7 +213,7 @@ restore_all_guest:
         movq  8(%rsp), %rcx           # RIP
 /* No special register assumptions. */
 iret_exit_to_guest:
-        andl  $~(X86_EFLAGS_IOPL|X86_EFLAGS_NT|X86_EFLAGS_VM),24(%rsp)
+        andl  $~(X86_EFLAGS_IOPL | X86_EFLAGS_VM), 24(%rsp)
         orl   $X86_EFLAGS_IF,24(%rsp)
         addq  $8,%rsp
 .Lft0:  iretq
-- 
2.11.0