Re: [patch V4 part 1 11/36] x86/kvm: Handle async page faults directly through do_page_fault()

Paolo Bonzini
2020-05-06T07:00:33Z

On 05/05/20 15:16, Thomas Gleixner wrote:
> 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>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Sean Christopherson <sean.j.christopherson@intel.com>
> ---
> 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
> @@ -1693,14 +1693,6 @@ 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
> @@ -1204,10 +1204,6 @@ 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;
> @@ -649,6 +649,9 @@ static void __init kvm_guest_init(void)
> if (kvm_para_has_feature(KVM_FEATURE_PV_EOI))
> apic_set_eoi_write(kvm_guest_apic_eoi_write);
>
> + if (kvm_para_has_feature(KVM_FEATURE_ASYNC_PF) && kvmapf)
> + static_branch_enable(&kvm_async_pf_enabled);
> +
> #ifdef CONFIG_SMP
> smp_ops.smp_prepare_cpus = kvm_smp_prepare_cpus;
> smp_ops.smp_prepare_boot_cpu = kvm_smp_prepare_boot_cpu;
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -988,7 +988,5 @@ 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>
> @@ -1523,6 +1524,24 @@ 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)))
>

Acked-by: Paolo Bonzini <pbonzini@redhat.com>