[patch 1/2] x86/kvm: Handle async page faults directly through do_page_fault()

Thomas Gleixner
2020-03-07T00:12:21Z

From: Andy Lutomirski <luto@kernel.org>

KVM overloads #PF to indicate two types of not-actually-page-fault
events. Right now, the KVM guest code intercepts them by modifying
the IDT and hooking the #PF vector. This makes the already fragile
fault code even harder to understand, and it also pollutes call
traces with async_page_fault and do_async_page_fault for normal page
faults.

Clean it up by moving the logic into do_page_fault() using a static
branch. This gets rid of the platform trap_init override mechanism
completely.

[ tglx: Fixed up 32bit, removed error code from the async functions and
massaged coding style ]

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
arch/x86/entry/entry_32.S | 8 --------
arch/x86/entry/entry_64.S | 4 ----
arch/x86/include/asm/kvm_para.h | 19 +++++++++++++++++--
arch/x86/include/asm/x86_init.h | 2 --
arch/x86/kernel/kvm.c | 39 +++++++++++++++++++++------------------
arch/x86/kernel/traps.c | 2 --
arch/x86/kernel/x86_init.c | 1 -
arch/x86/mm/fault.c | 19 +++++++++++++++++++
8 files changed, 57 insertions(+), 37 deletions(-)

--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -1698,14 +1698,6 @@ SYM_CODE_START(general_protection)
jmp common_exception
SYM_CODE_END(general_protection)

-#ifdef CONFIG_KVM_GUEST
-SYM_CODE_START(async_page_fault)
- ASM_CLAC
- pushl $do_async_page_fault
- jmp common_exception_read_cr2
-SYM_CODE_END(async_page_fault)
-#endif
-
SYM_CODE_START(rewind_stack_do_exit)
/* Prevent any naive code from trying to unwind to our caller. */
xorl %ebp, %ebp
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1202,10 +1202,6 @@ idtentry xendebug do_debug has_error_c
idtentry general_protection do_general_protection has_error_code=1
idtentry page_fault do_page_fault has_error_code=1 read_cr2=1

-#ifdef CONFIG_KVM_GUEST
-idtentry async_page_fault do_async_page_fault has_error_code=1 read_cr2=1
-#endif
-
#ifdef CONFIG_X86_MCE
idtentry machine_check do_mce has_error_code=0 paranoid=1
#endif
--- a/arch/x86/include/asm/kvm_para.h
+++ b/arch/x86/include/asm/kvm_para.h
@@ -91,8 +91,18 @@ unsigned int kvm_arch_para_hints(void);
void kvm_async_pf_task_wait(u32 token, int interrupt_kernel);
void kvm_async_pf_task_wake(u32 token);
u32 kvm_read_and_reset_pf_reason(void);
-extern void kvm_disable_steal_time(void);
-void do_async_page_fault(struct pt_regs *regs, unsigned long error_code, unsigned long address);
+void kvm_disable_steal_time(void);
+bool __kvm_handle_async_pf(struct pt_regs *regs, u32 token);
+
+DECLARE_STATIC_KEY_FALSE(kvm_async_pf_enabled);
+
+static __always_inline bool kvm_handle_async_pf(struct pt_regs *regs, u32 token)
+{
+ if (static_branch_unlikely(&kvm_async_pf_enabled))
+ return __kvm_handle_async_pf(regs, token);
+ else
+ return false;
+}

#ifdef CONFIG_PARAVIRT_SPINLOCKS
void __init kvm_spinlock_init(void);
@@ -130,6 +140,11 @@ static inline void kvm_disable_steal_tim
{
return;
}
+
+static inline bool kvm_handle_async_pf(struct pt_regs *regs, u32 token)
+{
+ return false;
+}
#endif

#endif /* _ASM_X86_KVM_PARA_H */
--- a/arch/x86/include/asm/x86_init.h
+++ b/arch/x86/include/asm/x86_init.h
@@ -50,14 +50,12 @@ struct x86_init_resources {
* @pre_vector_init: init code to run before interrupt vectors
* are set up.
* @intr_init: interrupt init code
- * @trap_init: platform specific trap setup
* @intr_mode_select: interrupt delivery mode selection
* @intr_mode_init: interrupt delivery mode setup
*/
struct x86_init_irqs {
void (*pre_vector_init)(void);
void (*intr_init)(void);
- void (*trap_init)(void);
void (*intr_mode_select)(void);
void (*intr_mode_init)(void);
};
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -35,6 +35,8 @@
#include <asm/tlb.h>
#include <asm/cpuidle_haltpoll.h>

+DEFINE_STATIC_KEY_FALSE(kvm_async_pf_enabled);
+
static int kvmapf = 1;

static int __init parse_no_kvmapf(char *arg)
@@ -242,25 +244,27 @@ u32 kvm_read_and_reset_pf_reason(void)
EXPORT_SYMBOL_GPL(kvm_read_and_reset_pf_reason);
NOKPROBE_SYMBOL(kvm_read_and_reset_pf_reason);

-dotraplinkage void
-do_async_page_fault(struct pt_regs *regs, unsigned long error_code, unsigned long address)
+bool __kvm_handle_async_pf(struct pt_regs *regs, u32 token)
{
+ /*
+ * If we get a page fault right here, the pf_reason seems likely
+ * to be clobbered. Bummer.
+ */
switch (kvm_read_and_reset_pf_reason()) {
default:
- do_page_fault(regs, error_code, address);
- break;
+ return false;
case KVM_PV_REASON_PAGE_NOT_PRESENT:
/* page is swapped out by the host. */
- kvm_async_pf_task_wait((u32)address, !user_mode(regs));
- break;
+ kvm_async_pf_task_wait(token, !user_mode(regs));
+ return true;
case KVM_PV_REASON_PAGE_READY:
rcu_irq_enter();
- kvm_async_pf_task_wake((u32)address);
+ kvm_async_pf_task_wake(token);
rcu_irq_exit();
- break;
+ return true;
}
}
-NOKPROBE_SYMBOL(do_async_page_fault);
+NOKPROBE_SYMBOL(__kvm_handle_async_pf);

static void __init paravirt_ops_setup(void)
{
@@ -306,7 +310,11 @@ static notrace void kvm_guest_apic_eoi_w
static void kvm_guest_cpu_init(void)
{
if (kvm_para_has_feature(KVM_FEATURE_ASYNC_PF) && kvmapf) {
- u64 pa = slow_virt_to_phys(this_cpu_ptr(&apf_reason));
+ u64 pa;
+
+ WARN_ON_ONCE(!static_branch_likely(&kvm_async_pf_enabled));
+
+ pa = slow_virt_to_phys(this_cpu_ptr(&apf_reason));

#ifdef CONFIG_PREEMPTION
pa |= KVM_ASYNC_PF_SEND_ALWAYS;
@@ -592,12 +600,6 @@ static int kvm_cpu_down_prepare(unsigned
}
#endif

-static void __init kvm_apf_trap_init(void)
-{
- update_intr_gate(X86_TRAP_PF, async_page_fault);
-}
-
-
static void kvm_flush_tlb_others(const struct cpumask *cpumask,
const struct flush_tlb_info *info)
{
@@ -632,8 +634,6 @@ static void __init kvm_guest_init(void)
register_reboot_notifier(&kvm_pv_reboot_nb);
for (i = 0; i < KVM_TASK_SLEEP_HASHSIZE; i++)
raw_spin_lock_init(&async_pf_sleepers[i].lock);
- if (kvm_para_has_feature(KVM_FEATURE_ASYNC_PF))
- x86_init.irqs.trap_init = kvm_apf_trap_init;

if (kvm_para_has_feature(KVM_FEATURE_STEAL_TIME)) {
has_steal_clock = 1;
@@ -670,6 +670,9 @@ static void __init kvm_guest_init(void)
* overcommitted.
*/
hardlockup_detector_disable();
+
+ if (kvm_para_has_feature(KVM_FEATURE_ASYNC_PF) && kvmapf)
+ static_branch_enable(&kvm_async_pf_enabled);
}

static noinline uint32_t __kvm_cpuid_base(void)
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -936,7 +936,5 @@ void __init trap_init(void)

idt_setup_ist_traps();

- x86_init.irqs.trap_init();
-
idt_setup_debugidt_traps();
}
--- a/arch/x86/kernel/x86_init.c
+++ b/arch/x86/kernel/x86_init.c
@@ -79,7 +79,6 @@ struct x86_init_ops x86_init __initdata
.irqs = {
.pre_vector_init = init_ISA_irqs,
.intr_init = native_init_IRQ,
- .trap_init = x86_init_noop,
.intr_mode_select = apic_intr_mode_select,
.intr_mode_init = apic_intr_mode_init
},
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -30,6 +30,7 @@
#include <asm/desc.h> /* store_idt(), ... */
#include <asm/cpu_entry_area.h> /* exception stack */
#include <asm/pgtable_areas.h> /* VMALLOC_START, ... */
+#include <asm/kvm_para.h> /* kvm_handle_async_pf */

#define CREATE_TRACE_POINTS
#include <asm/trace/exceptions.h>
@@ -1505,6 +1506,24 @@ do_page_fault(struct pt_regs *regs, unsi
unsigned long address)
{
prefetchw(&current->mm->mmap_sem);
+ /*
+ * KVM has two types of events that are, logically, interrupts, but
+ * are unfortunately delivered using the #PF vector. These events are
+ * "you just accessed valid memory, but the host doesn't have it right
+ * now, so I'll put you to sleep if you continue" and "that memory
+ * you tried to access earlier is available now."
+ *
+ * We are relying on the interrupted context being sane (valid RSP,
+ * relevant locks not held, etc.), which is fine as long as the
+ * interrupted context had IF=1. We are also relying on the KVM
+ * async pf type field and CR2 being read consistently instead of
+ * getting values from real and async page faults mixed up.
+ *
+ * Fingers crossed.
+ */
+ if (kvm_handle_async_pf(regs, (u32)address))
+ return;
+
trace_page_fault_entries(regs, hw_error_code, address);

if (unlikely(kmmio_fault(regs, address)))