[xen master] x86/boot: Document the ordering dependency of _svm_cpu_up()

[patchbot@xxxxxxx]

commit c2f15b0fff94ca71fe32792ea933fcf2a907111d
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Mon Feb 16 20:57:02 2026 +0000
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Tue Mar 3 15:15:54 2026 +0000

    x86/boot: Document the ordering dependency of _svm_cpu_up()
    
    Lets just say this took an unreasonable amount of time and effort to track
    down, when trying to move traps_init() earlier during boot.
    
    When the SYSCALL linkage MSRs are not configured ahead of _svm_cpu_up() on 
the
    BSP, context switch clobbers the later-set-up linkage with the 0's cached
    here.  Amongst other problems, this causes PV guest to enter at 0 in
    supervisor mode on the user stack.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/hvm/svm/svm.c | 14 ++++++++++++++
 xen/arch/x86/setup.c       |  2 +-
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/hvm/svm/svm.c b/xen/arch/x86/hvm/svm/svm.c
index 18ba837738..243c41fb13 100644
--- a/xen/arch/x86/hvm/svm/svm.c
+++ b/xen/arch/x86/hvm/svm/svm.c
@@ -1581,6 +1581,20 @@ static int _svm_cpu_up(bool bsp)
     /* Initialize OSVW bits to be used by guests */
     svm_host_osvw_init();
 
+    /*
+     * VMSAVE writes out the current full FS, GS, LDTR and TR segments, and
+     * the GS_SHADOW, SYSENTER and SYSCALL linkage MSRs.
+     *
+     * The segment data gets modified by the svm_load_segs() optimisation for
+     * PV context switches, but all values get reloaded at that point, as well
+     * as during context switch from SVM.
+     *
+     * If PV guests are available, it is critical that the SYSCALL linkage
+     * MSRs been configured at this juncture even in FRED mode.
+     */
+    if ( IS_ENABLED(CONFIG_PV) )
+        ASSERT(rdmsr(MSR_STAR) == XEN_MSR_STAR);
+
     svm_vmsave_pa(per_cpu(host_vmcb, cpu));
 
     return 0;
diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
index 27c63d1d97..675de3a649 100644
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -2078,7 +2078,7 @@ void asmlinkage __init noreturn __start_xen(void)
                                            &this_cpu(stubs).mfn);
     BUG_ON(!this_cpu(stubs.addr));
 
-    traps_init(); /* Needs stubs allocated. */
+    traps_init(); /* Needs stubs allocated, must be before presmp_initcalls. */
 
     cpu_init();
 
--
generated by git-patchbot for /home/xen/git/xen.git#master