[xen master] x86/traps: Introduce opt_fred

[patchbot@xxxxxxx]

commit 743fd27306b459b2c99c0e10f7a2c83fdd1f9365
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Sat Dec 28 16:39:55 2024 +0000
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Thu Aug 28 23:12:32 2025 +0100

    x86/traps: Introduce opt_fred
    
    ... disabled by default.  There is a lot of work before FRED can be enabled 
by
    default.
    
    One part of FRED, the LKGS (Load Kernel GS) instruction, is enumerated
    separately but expected to be mandatory as FRED disallows the SWAPGS
    instruction.  Normally, we'd have to check both CPUID bits, but Xen does not
    use GS like most other software, and can manage without LKGS.
    
    FRED formally removes the use of Ring1 and Ring2, meaning we cannot run 
32bit
    PV guests.  As already done with CET, disable PV32 if using FRED, except in
    shim mode where FRED is disabled by default.
    
    No functional change.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 docs/misc/xen-command-line.pandoc | 10 ++++++++++
 xen/arch/x86/include/asm/traps.h  |  4 ++++
 xen/arch/x86/traps-setup.c        | 36 ++++++++++++++++++++++++++++++++++++
 3 files changed, 50 insertions(+)

diff --git a/docs/misc/xen-command-line.pandoc 
b/docs/misc/xen-command-line.pandoc
index 3916cc81f6..4adcd7e762 100644
--- a/docs/misc/xen-command-line.pandoc
+++ b/docs/misc/xen-command-line.pandoc
@@ -1287,6 +1287,16 @@ requirement can be relaxed.  This option is particularly 
useful for nested
 virtualization, to allow the L1 hypervisor to use EPT even if the L0 hypervisor
 does not provide `VM_ENTRY_LOAD_GUEST_PAT`.
 
+### fred (x86)
+> `= <bool>`
+
+> Default: `false`
+
+Flexible Return and Event Delivery is an overhaul of interrupt, exception and
+system call handling, fixing many corner cases in the x86 architecture, and
+expected in hardware from 2025.  Support in Xen is a work in progress and
+disabled by default.
+
 ### gnttab
 > `= List of [ max-ver:<integer>, transitive=<bool>, transfer=<bool> ]`
 
diff --git a/xen/arch/x86/include/asm/traps.h b/xen/arch/x86/include/asm/traps.h
index 6ae451d3fc..73097e957d 100644
--- a/xen/arch/x86/include/asm/traps.h
+++ b/xen/arch/x86/include/asm/traps.h
@@ -7,6 +7,10 @@
 #ifndef ASM_TRAP_H
 #define ASM_TRAP_H
 
+#include <xen/types.h>
+
+extern int8_t opt_fred;
+
 void bsp_early_traps_init(void);
 void traps_init(void);
 void bsp_traps_reinit(void);
diff --git a/xen/arch/x86/traps-setup.c b/xen/arch/x86/traps-setup.c
index c89280270f..6e2af58ba0 100644
--- a/xen/arch/x86/traps-setup.c
+++ b/xen/arch/x86/traps-setup.c
@@ -9,6 +9,8 @@
 #include <asm/endbr.h>
 #include <asm/idt.h>
 #include <asm/msr.h>
+#include <asm/pv/domain.h>
+#include <asm/pv/shim.h>
 #include <asm/shstk.h>
 #include <asm/stubs.h>
 #include <asm/traps.h>
@@ -20,6 +22,9 @@ unsigned int __ro_after_init ler_msr;
 static bool __initdata opt_ler;
 boolean_param("ler", opt_ler);
 
+int8_t __ro_after_init opt_fred = 0;
+boolean_param("fred", opt_fred);
+
 void nocall entry_PF(void);
 void nocall lstar_enter(void);
 void nocall cstar_enter(void);
@@ -299,6 +304,37 @@ void __init traps_init(void)
     /* Replace early pagefault with real pagefault handler. */
     _update_gate_addr_lower(&bsp_idt[X86_EXC_PF], entry_PF);
 
+    /*
+     * Xen doesn't use GS like most software does, and doesn't need the LKGS
+     * instruction in order to manage PV guests.  No need to check for it.
+     */
+    if ( !cpu_has_fred )
+    {
+        if ( opt_fred == 1 )
+            printk(XENLOG_WARNING "FRED not available, ignoring\n");
+        opt_fred = 0;
+    }
+
+    if ( opt_fred == -1 )
+        opt_fred = !pv_shim;
+
+    if ( opt_fred )
+    {
+#ifdef CONFIG_PV32
+        if ( opt_pv32 )
+        {
+            opt_pv32 = 0;
+            printk(XENLOG_INFO "Disabling PV32 due to FRED\n");
+        }
+#endif
+        setup_force_cpu_cap(X86_FEATURE_XEN_FRED);
+        printk("Using FRED event delivery\n");
+    }
+    else
+    {
+        printk("Using IDT event delivery\n");
+    }
+
     load_system_tables();
 
     init_ler();
--
generated by git-patchbot for /home/xen/git/xen.git#master