Re: [patch V6 10/37] x86/entry: Switch XEN/PV hypercall entry to IDTENTRY

From: Thomas Gleixner
Date: Wed May 20 2020 - 15:17:41 EST

Andy Lutomirski <luto@xxxxxxxxxx> writes:
> Andrew Cooper pointed out that there is too much magic in Xen for this
> to work. So never mind.

:)

But you made me stare more at that stuff and I came up with a way
simpler solution. See below.

Thanks,

tglx

8<--------------

arch/x86/entry/common.c | 75 ++++++++++++++++++++++++++++++++++++++--
arch/x86/entry/entry_32.S | 17 ++++-----
arch/x86/entry/entry_64.S | 22 +++--------
arch/x86/include/asm/idtentry.h | 13 ++++++
arch/x86/xen/setup.c | 4 +-
arch/x86/xen/smp_pv.c | 3 +
arch/x86/xen/xen-asm_32.S | 12 +++---
arch/x86/xen/xen-asm_64.S | 2 -
arch/x86/xen/xen-ops.h | 1
drivers/xen/Makefile | 2 -
drivers/xen/preempt.c | 42 ----------------------
11 files changed, 115 insertions(+), 78 deletions(-)

--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -27,6 +27,9 @@
#include <linux/syscalls.h>
#include <linux/uaccess.h>

+#include <xen/xen-ops.h>
+#include <xen/events.h>
+
#include <asm/desc.h>
#include <asm/traps.h>
#include <asm/vdso.h>
@@ -35,6 +38,7 @@
#include <asm/nospec-branch.h>
#include <asm/io_bitmap.h>
#include <asm/syscall.h>
+#include <asm/irq_stack.h>

#define CREATE_TRACE_POINTS
#include <trace/events/syscalls.h>
@@ -539,7 +543,8 @@ void noinstr idtentry_enter(struct pt_re
}
}

-static __always_inline void __idtentry_exit(struct pt_regs *regs)
+static __always_inline void __idtentry_exit(struct pt_regs *regs,
+ bool may_sched)
{
lockdep_assert_irqs_disabled();

@@ -548,7 +553,7 @@ static __always_inline void __idtentry_e
prepare_exit_to_usermode(regs);
} else if (regs->flags & X86_EFLAGS_IF) {
/* Check kernel preemption, if enabled */
- if (IS_ENABLED(CONFIG_PREEMPTION)) {
+ if (IS_ENABLED(CONFIG_PREEMPTION) || may_resched) {
/*
* This needs to be done very carefully.
* idtentry_enter() invoked rcu_irq_enter(). This
@@ -612,5 +617,69 @@ static __always_inline void __idtentry_e
*/
void noinstr idtentry_exit(struct pt_regs *regs)
{
- __idtentry_exit(regs);
+ __idtentry_exit(regs, false);
+}
+
+#ifdef CONFIG_XEN_PV
+
+#ifndef CONFIG_PREEMPTION
+/*
+ * Some hypercalls issued by the toolstack can take many 10s of
+ * seconds. Allow tasks running hypercalls via the privcmd driver to
+ * be voluntarily preempted even if full kernel preemption is
+ * disabled.
+ *
+ * Such preemptible hypercalls are bracketed by
+ * xen_preemptible_hcall_begin() and xen_preemptible_hcall_end()
+ * calls.
+ */
+DEFINE_PER_CPU(bool, xen_in_preemptible_hcall);
+EXPORT_SYMBOL_GPL(xen_in_preemptible_hcall);
+
+/*
+ * In case of scheduling the flag must be cleared and restored after
+ * returning from schedule as the task might move to a different CPU.
+ */
+static __always_inline bool get_and_clear_inhcall(void)
+{
+ boot inhcall = __this_cpu_read(xen_in_preemptible_hcall);
+
+ __this_cpu_write(xen_in_preemptible_hcall, false);
+}
+
+static __always_inline void restore_inhcall(bool inhcall)
+{
+ __this_cpu_write(xen_in_preemptible_hcall, inhcall);
+}
+#else
+static __always_inline bool get_and_clear_inhcall(void) { return false; }
+static __always_inline void restore_inhcall(bool inhcall) { }
+#endif
+
+static void __xen_pv_evtchn_do_upcall(void)
+{
+ irq_enter_rcu();
+ inc_irq_stat(irq_hv_callback_count);
+
+ xen_hvm_evtchn_do_upcall();
+
+ irq_exit_rcu();
+}
+
+__visible noinstr void xen_pv_evtchn_do_upcall(struct pt_regs *regs)
+{
+ struct pt_regs *old_regs;
+ bool inhcall;
+
+ idtentry_enter(regs);
+ old_regs = set_irq_regs(regs);
+
+ run_on_irqstack(__xen_pv_evtchn_do_upcall, NULL, regs);
+
+ set_irq_regs(old_regs);
+
+ inhcall = get_and_clear_inhcall();
+ __idtentry_exit(regs, inhcall);
+ restore_inhcall(inhcall);
}
+#endif /* CONFIG_XEN_PV */
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -1298,7 +1298,10 @@ SYM_CODE_END(native_iret)
#endif

#ifdef CONFIG_XEN_PV
-SYM_FUNC_START(xen_hypervisor_callback)
+/*
+ * See comment in entry_64.S for further explanation
+ */
+SYM_FUNC_START(exc_xen_hypervisor_callback)
/*
* Check to see if we got the event in the critical
* region in xen_iret_direct, after we've reenabled
@@ -1315,14 +1318,11 @@ SYM_FUNC_START(xen_hypervisor_callback)
pushl $-1 /* orig_ax = -1 => not a system call */
SAVE_ALL
ENCODE_FRAME_POINTER
- TRACE_IRQS_OFF
+
mov %esp, %eax
- call xen_evtchn_do_upcall
-#ifndef CONFIG_PREEMPTION
- call xen_maybe_preempt_hcall
-#endif
- jmp ret_from_intr
-SYM_FUNC_END(xen_hypervisor_callback)
+ call xen_pv_evtchn_do_upcall
+ jmp handle_exception_return
+SYM_FUNC_END(exc_xen_hypervisor_callback)

/*
* Hypervisor uses this for application faults while it executes.
@@ -1464,6 +1464,7 @@ SYM_CODE_START_LOCAL_NOALIGN(handle_exce
movl %esp, %eax # pt_regs pointer
CALL_NOSPEC edi

+handle_exception_return:
#ifdef CONFIG_VM86
movl PT_EFLAGS(%esp), %eax # mix EFLAGS and CS
movb PT_CS(%esp), %al
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1067,10 +1067,6 @@ apicinterrupt IRQ_WORK_VECTOR irq_work

idtentry X86_TRAP_PF page_fault do_page_fault has_error_code=1

-#ifdef CONFIG_XEN_PV
-idtentry 512 /* dummy */ hypervisor_callback xen_do_hypervisor_callback has_error_code=0
-#endif
-
/*
* Reload gs selector with exception handling
* edi: new selector
@@ -1158,9 +1154,10 @@ SYM_FUNC_END(asm_call_on_stack)
* So, on entry to the handler we detect whether we interrupted an
* existing activation in its critical region -- if so, we pop the current
* activation and restart the handler using the previous one.
+ *
+ * C calling convention: exc_xen_hypervisor_callback(struct *pt_regs)
*/
-/* do_hypervisor_callback(struct *pt_regs) */
-SYM_CODE_START_LOCAL(xen_do_hypervisor_callback)
+SYM_CODE_START_LOCAL(exc_xen_hypervisor_callback)

/*
* Since we don't modify %rdi, evtchn_do_upall(struct *pt_regs) will
@@ -1170,15 +1167,10 @@ SYM_CODE_START_LOCAL(xen_do_hypervisor_c
movq %rdi, %rsp /* we don't return, adjust the stack frame */
UNWIND_HINT_REGS

- ENTER_IRQ_STACK old_rsp=%r10
- call xen_evtchn_do_upcall
- LEAVE_IRQ_STACK
-
-#ifndef CONFIG_PREEMPTION
- call xen_maybe_preempt_hcall
-#endif
- jmp error_exit
-SYM_CODE_END(xen_do_hypervisor_callback)
+ call xen_pv_evtchn_do_upcall
+
+ jmp error_return
+SYM_CODE_END(exc_xen_hypervisor_callback)

/*
* Hypervisor uses this for application faults while it executes.
--- a/arch/x86/include/asm/idtentry.h
+++ b/arch/x86/include/asm/idtentry.h
@@ -332,6 +332,13 @@ static __always_inline void __##func(str
* This avoids duplicate defines and ensures that everything is consistent.
*/

+/*
+ * Dummy trap number so the low level ASM macro vector number checks do not
+ * match which results in emitting plain IDTENTRY stubs without bells and
+ * whistels.
+ */
+#define X86_TRAP_OTHER 0xFFFF
+
/* Simple exception entry points. No hardware error code */
DECLARE_IDTENTRY(X86_TRAP_DE, exc_divide_error);
DECLARE_IDTENTRY(X86_TRAP_OF, exc_overflow);
@@ -371,4 +378,10 @@ DECLARE_IDTENTRY_XEN(X86_TRAP_DB, debug)
/* #DF */
DECLARE_IDTENTRY_DF(X86_TRAP_DF, exc_double_fault);

+#ifdef CONFIG_XEN_PV
+DECLARE_IDTENTRY(X86_TRAP_OTHER, exc_xen_hypervisor_callback);
+#endif
+
+#undef X86_TRAP_OTHER
+
#endif
--- a/arch/x86/xen/setup.c
+++ b/arch/x86/xen/setup.c
@@ -20,6 +20,7 @@
#include <asm/setup.h>
#include <asm/acpi.h>
#include <asm/numa.h>
+#include <asm/idtentry.h>
#include <asm/xen/hypervisor.h>
#include <asm/xen/hypercall.h>

@@ -993,7 +994,8 @@ static void __init xen_pvmmu_arch_setup(
HYPERVISOR_vm_assist(VMASST_CMD_enable,
VMASST_TYPE_pae_extended_cr3);

- if (register_callback(CALLBACKTYPE_event, xen_hypervisor_callback) ||
+ if (register_callback(CALLBACKTYPE_event,
+ xen_asm_exc_xen_hypervisor_callback) ||
register_callback(CALLBACKTYPE_failsafe, xen_failsafe_callback))
BUG();

--- a/arch/x86/xen/smp_pv.c
+++ b/arch/x86/xen/smp_pv.c
@@ -27,6 +27,7 @@
#include <asm/paravirt.h>
#include <asm/desc.h>
#include <asm/pgtable.h>
+#include <asm/idtentry.h>
#include <asm/cpu.h>

#include <xen/interface/xen.h>
@@ -347,7 +348,7 @@ cpu_initialize_context(unsigned int cpu,
ctxt->gs_base_kernel = per_cpu_offset(cpu);
#endif
ctxt->event_callback_eip =
- (unsigned long)xen_hypervisor_callback;
+ (unsigned long)xen_asm_exc_xen_hypervisor_callback;
ctxt->failsafe_callback_eip =
(unsigned long)xen_failsafe_callback;
per_cpu(xen_cr3, cpu) = __pa(swapper_pg_dir);
--- a/arch/x86/xen/xen-asm_32.S
+++ b/arch/x86/xen/xen-asm_32.S
@@ -93,7 +93,7 @@ SYM_CODE_START(xen_iret)

/*
* If there's something pending, mask events again so we can
- * jump back into xen_hypervisor_callback. Otherwise do not
+ * jump back into exc_xen_hypervisor_callback. Otherwise do not
* touch XEN_vcpu_info_mask.
*/
jne 1f
@@ -113,7 +113,7 @@ SYM_CODE_START(xen_iret)
* Events are masked, so jumping out of the critical region is
* OK.
*/
- je xen_hypervisor_callback
+ je asm_exc_xen_hypervisor_callback

1: iret
xen_iret_end_crit:
@@ -127,7 +127,7 @@ SYM_CODE_END(xen_iret)
.globl xen_iret_start_crit, xen_iret_end_crit

/*
- * This is called by xen_hypervisor_callback in entry_32.S when it sees
+ * This is called by exc_xen_hypervisor_callback in entry_32.S when it sees
* that the EIP at the time of interrupt was between
* xen_iret_start_crit and xen_iret_end_crit.
*
@@ -144,7 +144,7 @@ SYM_CODE_END(xen_iret)
* eflags }
* cs } nested exception info
* eip }
- * return address : (into xen_hypervisor_callback)
+ * return address : (into asm_exc_xen_hypervisor_callback)
*
* In order to deliver the nested exception properly, we need to discard the
* nested exception frame such that when we handle the exception, we do it
@@ -152,7 +152,8 @@ SYM_CODE_END(xen_iret)
*
* The only caveat is that if the outer eax hasn't been restored yet (i.e.
* it's still on stack), we need to restore its value here.
- */
+*/
+.pushsection .noinstr.text, "ax"
SYM_CODE_START(xen_iret_crit_fixup)
/*
* Paranoia: Make sure we're really coming from kernel space.
@@ -181,3 +182,4 @@ SYM_CODE_START(xen_iret_crit_fixup)
2:
ret
SYM_CODE_END(xen_iret_crit_fixup)
+.popsection
--- a/arch/x86/xen/xen-asm_64.S
+++ b/arch/x86/xen/xen-asm_64.S
@@ -54,7 +54,7 @@ xen_pv_trap asm_exc_simd_coprocessor_err
#ifdef CONFIG_IA32_EMULATION
xen_pv_trap entry_INT80_compat
#endif
-xen_pv_trap hypervisor_callback
+xen_pv_trap asm_exc_xen_hypervisor_callback

__INIT
SYM_CODE_START(xen_early_idt_handler_array)
--- a/arch/x86/xen/xen-ops.h
+++ b/arch/x86/xen/xen-ops.h
@@ -8,7 +8,6 @@
#include <xen/xen-ops.h>

/* These are code, but not functions. Defined in entry.S */
-extern const char xen_hypervisor_callback[];
extern const char xen_failsafe_callback[];

void xen_sysenter_target(void);
--- a/drivers/xen/Makefile
+++ b/drivers/xen/Makefile
@@ -1,6 +1,6 @@
# SPDX-License-Identifier: GPL-2.0
obj-$(CONFIG_HOTPLUG_CPU) += cpu_hotplug.o
-obj-y += grant-table.o features.o balloon.o manage.o preempt.o time.o
+obj-y += grant-table.o features.o balloon.o manage.o time.o
obj-y += mem-reservation.o
obj-y += events/
obj-y += xenbus/
--- a/drivers/xen/preempt.c
+++ /dev/null
@@ -1,42 +0,0 @@
-// SPDX-License-Identifier: GPL-2.0-or-later
-/*
- * Preemptible hypercalls
- *
- * Copyright (C) 2014 Citrix Systems R&D ltd.
- */
-
-#include <linux/sched.h>
-#include <xen/xen-ops.h>
-
-#ifndef CONFIG_PREEMPTION
-
-/*
- * Some hypercalls issued by the toolstack can take many 10s of
- * seconds. Allow tasks running hypercalls via the privcmd driver to
- * be voluntarily preempted even if full kernel preemption is
- * disabled.
- *
- * Such preemptible hypercalls are bracketed by
- * xen_preemptible_hcall_begin() and xen_preemptible_hcall_end()
- * calls.
- */
-
-DEFINE_PER_CPU(bool, xen_in_preemptible_hcall);
-EXPORT_SYMBOL_GPL(xen_in_preemptible_hcall);
-
-asmlinkage __visible void xen_maybe_preempt_hcall(void)
-{
- if (unlikely(__this_cpu_read(xen_in_preemptible_hcall)
- && need_resched())) {
- /*
- * Clear flag as we may be rescheduled on a different
- * cpu.
- */
- __this_cpu_write(xen_in_preemptible_hcall, false);
- local_irq_enable();
- cond_resched();
- local_irq_disable();
- __this_cpu_write(xen_in_preemptible_hcall, true);
- }
-}
-#endif /* CONFIG_PREEMPTION */