KVM: x86: hardware task switching support

This emulates the x86 hardware task switch mechanism in software, as it is
unsupported by either vmx or svm.  It allows operating systems which use it,
like freedos, to run as kvm guests.
Signed-off-by: Izik Eidus <izike@qumranet.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>

arch/x86/kvm/svm.c

@@ -1112,9 +1112,18 @@ static int invalid_op_interception(struct vcpu_svm *svm,
 static int task_switch_interception(struct vcpu_svm *svm,
 				    struct kvm_run *kvm_run)
 {
-	pr_unimpl(&svm->vcpu, "%s: task switch is unsupported\n", __func__);
-	kvm_run->exit_reason = KVM_EXIT_UNKNOWN;
-	return 0;
+	u16 tss_selector;
+
+	tss_selector = (u16)svm->vmcb->control.exit_info_1;
+	if (svm->vmcb->control.exit_info_2 &
+	    (1ULL << SVM_EXITINFOSHIFT_TS_REASON_IRET))
+		return kvm_task_switch(&svm->vcpu, tss_selector,
+				       TASK_SWITCH_IRET);
+	if (svm->vmcb->control.exit_info_2 &
+	    (1ULL << SVM_EXITINFOSHIFT_TS_REASON_JMP))
+		return kvm_task_switch(&svm->vcpu, tss_selector,
+				       TASK_SWITCH_JMP);
+	return kvm_task_switch(&svm->vcpu, tss_selector, TASK_SWITCH_CALL);
 }
 
 static int cpuid_interception(struct vcpu_svm *svm, struct kvm_run *kvm_run)

arch/x86/kvm/svm.h

@@ -238,6 +238,9 @@ struct __attribute__ ((__packed__)) vmcb {
 #define SVM_EXITINTINFO_VALID SVM_EVTINJ_VALID
 #define SVM_EXITINTINFO_VALID_ERR SVM_EVTINJ_VALID_ERR
 
+#define SVM_EXITINFOSHIFT_TS_REASON_IRET 36
+#define SVM_EXITINFOSHIFT_TS_REASON_JMP 38
+
 #define	SVM_EXIT_READ_CR0 	0x000
 #define	SVM_EXIT_READ_CR3 	0x003
 #define	SVM_EXIT_READ_CR4 	0x004

arch/x86/kvm/tss.h

+#ifndef __TSS_SEGMENT_H
+#define __TSS_SEGMENT_H
+
+struct tss_segment_32 {
+	u32 prev_task_link;
+	u32 esp0;
+	u32 ss0;
+	u32 esp1;
+	u32 ss1;
+	u32 esp2;
+	u32 ss2;
+	u32 cr3;
+	u32 eip;
+	u32 eflags;
+	u32 eax;
+	u32 ecx;
+	u32 edx;
+	u32 ebx;
+	u32 esp;
+	u32 ebp;
+	u32 esi;
+	u32 edi;
+	u32 es;
+	u32 cs;
+	u32 ss;
+	u32 ds;
+	u32 fs;
+	u32 gs;
+	u32 ldt_selector;
+	u16 t;
+	u16 io_map;
+};
+
+struct tss_segment_16 {
+	u16 prev_task_link;
+	u16 sp0;
+	u16 ss0;
+	u16 sp1;
+	u16 ss1;
+	u16 sp2;
+	u16 ss2;
+	u16 ip;
+	u16 flag;
+	u16 ax;
+	u16 cx;
+	u16 dx;
+	u16 bx;
+	u16 sp;
+	u16 bp;
+	u16 si;
+	u16 di;
+	u16 es;
+	u16 cs;
+	u16 ss;
+	u16 ds;
+	u16 ldt;
+};
+
+#endif

arch/x86/kvm/vmx.c

@@ -2249,6 +2249,20 @@ static int handle_apic_access(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
 	return 1;
 }
 
+static int handle_task_switch(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
+{
+	unsigned long exit_qualification;
+	u16 tss_selector;
+	int reason;
+
+	exit_qualification = vmcs_readl(EXIT_QUALIFICATION);
+
+	reason = (u32)exit_qualification >> 30;
+	tss_selector = exit_qualification;
+
+	return kvm_task_switch(vcpu, tss_selector, reason);
+}
+
 /*
  * The exit handlers return 1 if the exit was handled fully and guest execution
  * may resume.  Otherwise they set the kvm_run parameter to indicate what needs
@@ -2271,6 +2285,7 @@ static int (*kvm_vmx_exit_handlers[])(struct kvm_vcpu *vcpu,
 	[EXIT_REASON_TPR_BELOW_THRESHOLD]     = handle_tpr_below_threshold,
 	[EXIT_REASON_APIC_ACCESS]             = handle_apic_access,
 	[EXIT_REASON_WBINVD]                  = handle_wbinvd,
+	[EXIT_REASON_TASK_SWITCH]             = handle_task_switch,
 };
 
 static const int kvm_vmx_max_exit_handlers =

include/asm-x86/kvm_host.h

@@ -492,6 +492,8 @@ int emulator_get_dr(struct x86_emulate_ctxt *ctxt, int dr,
 int emulator_set_dr(struct x86_emulate_ctxt *ctxt, int dr,
 		    unsigned long value);
 
+int kvm_task_switch(struct kvm_vcpu *vcpu, u16 tss_selector, int reason);
+
 void kvm_set_cr0(struct kvm_vcpu *vcpu, unsigned long cr0);
 void kvm_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr0);
 void kvm_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr0);
@@ -657,4 +659,11 @@ static inline void kvm_inject_gp(struct kvm_vcpu *vcpu, u32 error_code)
 #define RMODE_TSS_SIZE							\
 	(TSS_BASE_SIZE + TSS_REDIRECTION_SIZE + TSS_IOPB_SIZE + 1)
 
+enum {
+	TASK_SWITCH_CALL = 0,
+	TASK_SWITCH_IRET = 1,
+	TASK_SWITCH_JMP = 2,
+	TASK_SWITCH_GATE = 3,
+};
+
 #endif