Re: [PATCH 2/3] x86/split_lock: Refactor and export handle_user_split_lock() for KVM
From: Thomas Gleixner
Date: Thu Apr 02 2020 - 15:06:54 EST
Sean Christopherson <sean.j.christopherson@xxxxxxxxx> writes:
> On Thu, Apr 02, 2020 at 07:01:56PM +0200, Thomas Gleixner wrote:
>> > static inline void __init cpu_set_core_cap_bits(struct cpuinfo_x86 *c) {}
>> > static inline void switch_to_sld(unsigned long tifn) {}
>> > -static inline bool handle_user_split_lock(struct pt_regs *regs, long error_code)
>> > +static inline bool handle_user_split_lock(unsigned long ip)
>>
>> This is necessary because VMX can be compiled without CPU_SUP_INTEL?
>
> Ya, it came about when cleaning up the IA32_FEATURE_CONTROL MSR handling
> to consolidate duplicate code.
>
> config KVM_INTEL
> tristate "KVM for Intel (and compatible) processors support"
> depends on KVM && IA32_FEAT_CTL
>
> config IA32_FEAT_CTL
> def_bool y
> depends on CPU_SUP_INTEL || CPU_SUP_CENTAUR || CPU_SUP_ZHAOXIN
Ah, indeed. So something like the below would make sense. Hmm?
Of course that can be mangled into Xiaoyao's patches, I'm not worried
about my patch count :)
Aside of that I really wish Intel HW folks had indicated the source of
the #AC via the error code. It can only be 0 or 1 for the regular #AC so
there would have been 31 bits to chose from.
Thanks,
tglx
8<----------------
--- a/arch/x86/include/asm/cpu.h
+++ b/arch/x86/include/asm/cpu.h
@@ -43,14 +43,14 @@ unsigned int x86_stepping(unsigned int s
#ifdef CONFIG_CPU_SUP_INTEL
extern void __init cpu_set_core_cap_bits(struct cpuinfo_x86 *c);
extern void switch_to_sld(unsigned long tifn);
-extern bool handle_user_split_lock(struct pt_regs *regs, long error_code);
+extern int handle_ac_split_lock(unsigned long ip);
extern void split_lock_validate_module_text(struct module *me, void *text, void *text_end);
#else
static inline void __init cpu_set_core_cap_bits(struct cpuinfo_x86 *c) {}
static inline void switch_to_sld(unsigned long tifn) {}
-static inline bool handle_user_split_lock(struct pt_regs *regs, long error_code)
+static int handle_ac_split_lock(unsigned long ip)
{
- return false;
+ return -ENOSYS;
}
static inline void split_lock_validate_module_text(struct module *me, void *text, void *text_end) {}
#endif
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -1102,13 +1102,20 @@ static void split_lock_init(void)
split_lock_verify_msr(sld_state != sld_off);
}
-bool handle_user_split_lock(struct pt_regs *regs, long error_code)
+int handle_ac_split_lock(unsigned long ip)
{
- if ((regs->flags & X86_EFLAGS_AC) || sld_state == sld_fatal)
- return false;
+ switch (sld_state) {
+ case sld_warn:
+ break;
+ case sld_off:
+ pr_warn_once("#AC: Spurious trap at address: 0x%lx\n", ip);
+ return -ENOSYS;
+ case sld_fatal:
+ return -EFAULT;
+ }
pr_warn_ratelimited("#AC: %s/%d took a split_lock trap at address: 0x%lx\n",
- current->comm, current->pid, regs->ip);
+ current->comm, current->pid, ip);
/*
* Disable the split lock detection for this task so it can make
@@ -1117,8 +1124,9 @@ bool handle_user_split_lock(struct pt_re
*/
sld_update_msr(false);
set_tsk_thread_flag(current, TIF_SLD);
- return true;
+ return 0;
}
+EXPORT_SYMBOL_GPL(handle_ac_split_lock);
/*
* This function is called only when switching between tasks with
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -304,7 +304,7 @@ dotraplinkage void do_alignment_check(st
local_irq_enable();
- if (handle_user_split_lock(regs, error_code))
+ if (!(regs->flags & X86_EFLAGS_AC) && !handle_ac_split_lock(regs->ip))
return;
do_trap(X86_TRAP_AC, SIGBUS, "alignment check", regs,
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -65,6 +65,7 @@
MODULE_AUTHOR("Qumranet");
MODULE_LICENSE("GPL");
+MODULE_INFO(sld_safe, "Y");
#ifdef MODULE
static const struct x86_cpu_id vmx_cpu_id[] = {
@@ -4623,6 +4624,22 @@ static int handle_machine_check(struct k
return 1;
}
+static bool guest_handles_ac(struct kvm_vcpu *vcpu)
+{
+ /*
+ * If guest has alignment checking enabled in CR0 and activated in
+ * eflags, then the #AC originated from CPL3 and the guest is able
+ * to handle it. It does not matter whether this is a regular or
+ * a split lock operation induced #AC.
+ */
+ if (vmx_get_cpl(vcpu) == 3 && kvm_read_cr0_bits(vcpu, X86_CR0_AM) &&
+ kvm_get_rflags(vcpu) & X86_EFLAGS_AC)
+ return true;
+
+ /* Add guest SLD handling checks here once it's supported */
+ return false;
+}
+
static int handle_exception_nmi(struct kvm_vcpu *vcpu)
{
struct vcpu_vmx *vmx = to_vmx(vcpu);
@@ -4630,6 +4647,7 @@ static int handle_exception_nmi(struct k
u32 intr_info, ex_no, error_code;
unsigned long cr2, rip, dr6;
u32 vect_info;
+ int err;
vect_info = vmx->idt_vectoring_info;
intr_info = vmx->exit_intr_info;
@@ -4688,9 +4706,6 @@ static int handle_exception_nmi(struct k
return handle_rmode_exception(vcpu, ex_no, error_code);
switch (ex_no) {
- case AC_VECTOR:
- kvm_queue_exception_e(vcpu, AC_VECTOR, error_code);
- return 1;
case DB_VECTOR:
dr6 = vmcs_readl(EXIT_QUALIFICATION);
if (!(vcpu->guest_debug &
@@ -4719,6 +4734,29 @@ static int handle_exception_nmi(struct k
kvm_run->debug.arch.pc = vmcs_readl(GUEST_CS_BASE) + rip;
kvm_run->debug.arch.exception = ex_no;
break;
+ case AC_VECTOR:
+ if (guest_handles_ac(vcpu)) {
+ kvm_queue_exception_e(vcpu, AC_VECTOR, error_code);
+ return 1;
+ }
+ /*
+ * Handle #AC caused by split lock detection. If the host
+ * mode is sld_warn, then it warns, marks current with
+ * TIF_SLD and disables split lock detection. So the guest
+ * can just continue.
+ *
+ * If the host mode is fatal, the handling code warned. Let
+ * qemu kill itself.
+ *
+ * If the host mode is off, then this #AC is bonkers and
+ * something is badly wrong. Let it fail as well.
+ */
+ err = handle_ac_split_lock(kvm_rip_read(vcpu));
+ if (!err)
+ return 1;
+ /* Propagate the error type to user space */
+ error_code = err == -EFAULT ? 0x100 : 0x200;
+ fallthrough;
default:
kvm_run->exit_reason = KVM_EXIT_EXCEPTION;
kvm_run->ex.exception = ex_no;