[PATCH v2 03/23] x86/traps: Introduce opt_fred

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

... 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 is 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 the LKGS instruction.

FRED formally removes the use of Ring1 and Ring2, meaning we cannot run 32bit
PV guests.  Therefore, don't enable FRED by default in shim mode.  OTOH, if
FRED is active, then PV32 needs disabling like with CET.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>

v2:
 * Fix check for warning.
 * Drop check for LKGS.
---
 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 a75b6c930195..25cebdc1110f 100644
--- a/docs/misc/xen-command-line.pandoc
+++ b/docs/misc/xen-command-line.pandoc
@@ -1284,6 +1284,16 @@ 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 6ae451d3fc70..73097e957d05 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 c89280270fbb..6e2af58ba0a5 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();
-- 
2.39.5