[xen staging] x86/traps: Set MSR_PL0_SSP in load_system_tables()

[patchbot@xxxxxxx]

commit 667f90711b058cc9306232ab4d2215f41719cb0b
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Mon May 26 18:48:26 2025 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Mon Sep 1 19:54:21 2025 +0100

    x86/traps: Set MSR_PL0_SSP in load_system_tables()
    
    FRED and IDT differ by a Supervisor Token on the base of the shstk.  This
    means that the value they load into MSR_PL0_SSP differs by 8.
    
    s3_resume() in particular has logic which is otherwise invariant of FRED 
mode,
    and must not clobber a FRED MSR_PL0_SSP with an IDT one.
    
    This also simplifies the AP path too.
    
    No functional change.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/acpi/wakeup_prot.S |  9 ---------
 xen/arch/x86/boot/x86_64.S      | 12 +++---------
 xen/arch/x86/setup.c            |  2 --
 xen/arch/x86/traps-setup.c      |  2 ++
 4 files changed, 5 insertions(+), 20 deletions(-)

diff --git a/xen/arch/x86/acpi/wakeup_prot.S b/xen/arch/x86/acpi/wakeup_prot.S
index fceb4ca353..ba0bd77806 100644
--- a/xen/arch/x86/acpi/wakeup_prot.S
+++ b/xen/arch/x86/acpi/wakeup_prot.S
@@ -104,15 +104,6 @@ LABEL(s3_resume)
          */
         mov     saved_ssp(%rip), %rdi
 
-        /* Calculate MSR_PL0_SSP from SSP. */
-        mov     $MSR_PL0_SSP, %ecx
-        mov     %rdi, %rdx
-        shr     $32, %rdx
-        mov     %edi, %eax
-        and     $~(STACK_SIZE - 1), %eax
-        or      $(PRIMARY_SHSTK_SLOT + 1) * PAGE_SIZE - 8, %eax
-        wrmsr
-
         /*
          * A Restore Token's value is &token + 8 + 64BIT (bit 0).
          * We want to put this on the shstk at SSP - 8.
diff --git a/xen/arch/x86/boot/x86_64.S b/xen/arch/x86/boot/x86_64.S
index 3a5ad27644..11a7e9d3bd 100644
--- a/xen/arch/x86/boot/x86_64.S
+++ b/xen/arch/x86/boot/x86_64.S
@@ -65,17 +65,11 @@ ENTRY(__high_start)
         or      $(PRIMARY_SHSTK_SLOT + 1) * PAGE_SIZE - 8, %rdx
 
         /*
-         * Write a new supervisor token.  Doesn't matter on boot, but for S3
-         * resume this clears the busy bit.
+         * Write a new Supervisor Token.  It doesn't matter the first time a
+         * CPU boots, but for S3 resume or hotplug this clears the busy bit so
+         * SETSSBSY can set it again.
          */
         wrssq   %rdx, (%rdx)
-
-        /* Point MSR_PL0_SSP at the token. */
-        mov     $MSR_PL0_SSP, %ecx
-        mov     %edx, %eax
-        shr     $32, %rdx
-        wrmsr
-
         setssbsy
 
 #endif /* CONFIG_XEN_SHSTK */
diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
index 6c81841426..a810bdf6d3 100644
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -907,8 +907,6 @@ static void __init noreturn reinit_bsp_stack(void)
 
     if ( cpu_has_xen_shstk )
     {
-        wrmsrl(MSR_PL0_SSP,
-               (unsigned long)stack + (PRIMARY_SHSTK_SLOT + 1) * PAGE_SIZE - 
8);
         wrmsrl(MSR_S_CET, xen_msr_s_cet_value());
         asm volatile ("setssbsy" ::: "memory");
     }
diff --git a/xen/arch/x86/traps-setup.c b/xen/arch/x86/traps-setup.c
index 6e2af58ba0..d77be8f839 100644
--- a/xen/arch/x86/traps-setup.c
+++ b/xen/arch/x86/traps-setup.c
@@ -92,6 +92,7 @@ static void load_system_tables(void)
     {
         volatile uint64_t *ist_ssp = tss_page->ist_ssp;
         unsigned long
+            ssp = stack_top + (PRIMARY_SHSTK_SLOT + 1) * PAGE_SIZE - 8,
             mce_ssp = stack_top + (IST_MCE * IST_SHSTK_SIZE) - 8,
             nmi_ssp = stack_top + (IST_NMI * IST_SHSTK_SIZE) - 8,
             db_ssp  = stack_top + (IST_DB  * IST_SHSTK_SIZE) - 8,
@@ -118,6 +119,7 @@ static void load_system_tables(void)
         }
 
         wrmsrns(MSR_ISST, (unsigned long)ist_ssp);
+        wrmsrns(MSR_PL0_SSP, (unsigned long)ssp);
     }
 
     _set_tssldt_desc(gdt + TSS_ENTRY, (unsigned long)tss,
--
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