[PATCH for-next] xen/pv: Construct d0v0's GDT properly

[Andrew Cooper <andrew.cooper3@citrix.com>]

c/s cf6d39f8199 "x86/PV: properly populate descriptor tables" changed the GDT
to reference zero_page for intermediate frames between the guest and Xen
frames.

Because dom0_construct_pv() doesn't call arch_set_info_guest(), some bits of
initialisation are missed, including the pv_destroy_gdt() which initially
fills the references to zero_page.

In practice, this means there is a window between starting and the first call
to HYPERCALL_set_gdt() were lar/lsl/verr/verw suffer non-architectural
behaviour.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
CC: Jan Beulich <JBeulich@suse.com>

This probably wants backporting to Xen 4.7 and later.
---
 xen/arch/x86/pv/dom0_build.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/xen/arch/x86/pv/dom0_build.c b/xen/arch/x86/pv/dom0_build.c
index 8ad7e3d..984a94a 100644
--- a/xen/arch/x86/pv/dom0_build.c
+++ b/xen/arch/x86/pv/dom0_build.c
@@ -18,6 +18,7 @@
 #include <asm/bzimage.h>
 #include <asm/dom0_build.h>
 #include <asm/page.h>
+#include <asm/pv/mm.h>
 #include <asm/setup.h>
 
 #include "mm.h"
@@ -867,6 +868,13 @@ int __init dom0_construct_pv(struct domain *d,
     regs->rsi = vstartinfo_start;
     regs->eflags = X86_EFLAGS_IF;
 
+    /*
+     * We don't call arch_set_info_guest(), so some initialisation needs doing
+     * by hand:
+     *  - Reset the GDT to reference zero_page
+     */
+    pv_destroy_gdt(v);
+
     if ( test_bit(XENFEAT_supervisor_mode_kernel, parms.f_required) )
         panic("Dom0 requires supervisor-mode execution");
 
-- 
2.1.4


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