KVM: x86 emulator: add syscall emulation

Handle #UD intercept of the syscall instruction in 32bit compat mode on
an Intel host.
Setup the segment descriptors for CS and SS and the EIP/ESP registers
according to the manual. Save the RIP and EFLAGS to the correct registers.

[avi: fix build on i386 due to missing R11]

Signed-off-by: Christoph Egger <christoph.egger@amd.com>
Signed-off-by: Andre Przywara <andre.przywara@amd.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

1 files changed, 83 insertions, 1 deletions

diff --git a/arch/x86/kvm/x86_emulate.c b/arch/x86/kvm/x86_emulate.c
index b0da29d7403..4d7256da59d 100644
--- a/arch/x86/kvm/x86_emulate.c
+++ b/arch/x86/kvm/x86_emulate.c
@@ -1397,6 +1397,85 @@ static void toggle_interruptibility(struct x86_emulate_ctxt *ctxt, u32 mask)
                 ctxt->interruptibility = mask;
 }
 
+static inline void
+setup_syscalls_segments(struct x86_emulate_ctxt *ctxt,
+        struct kvm_segment *cs, struct kvm_segment *ss)
+{
+        memset(cs, 0, sizeof(struct kvm_segment));
+        kvm_x86_ops->get_segment(ctxt->vcpu, cs, VCPU_SREG_CS);
+        memset(ss, 0, sizeof(struct kvm_segment));
+
+        cs->l = 0;              /* will be adjusted later */
+        cs->base = 0;           /* flat segment */
+        cs->g = 1;              /* 4kb granularity */
+        cs->limit = 0xffffffff; /* 4GB limit */
+        cs->type = 0x0b;        /* Read, Execute, Accessed */
+        cs->s = 1;
+        cs->dpl = 0;            /* will be adjusted later */
+        cs->present = 1;
+        cs->db = 1;
+
+        ss->unusable = 0;
+        ss->base = 0;           /* flat segment */
+        ss->limit = 0xffffffff; /* 4GB limit */
+        ss->g = 1;              /* 4kb granularity */
+        ss->s = 1;
+        ss->type = 0x03;        /* Read/Write, Accessed */
+        ss->db = 1;             /* 32bit stack segment */
+        ss->dpl = 0;
+        ss->present = 1;
+}
+
+static int
+emulate_syscall(struct x86_emulate_ctxt *ctxt)
+{
+        struct decode_cache *c = &ctxt->decode;
+        struct kvm_segment cs, ss;
+        u64 msr_data;
+
+        /* syscall is not available in real mode */
+        if (c->lock_prefix || ctxt->mode == X86EMUL_MODE_REAL
+                || !(ctxt->vcpu->arch.cr0 & X86_CR0_PE))
+                return -1;
+
+        setup_syscalls_segments(ctxt, &cs, &ss);
+
+        kvm_x86_ops->get_msr(ctxt->vcpu, MSR_STAR, &msr_data);
+        msr_data >>= 32;
+        cs.selector = (u16)(msr_data & 0xfffc);
+        ss.selector = (u16)(msr_data + 8);
+
+        if (is_long_mode(ctxt->vcpu)) {
+                cs.db = 0;
+                cs.l = 1;
+        }
+        kvm_x86_ops->set_segment(ctxt->vcpu, &cs, VCPU_SREG_CS);
+        kvm_x86_ops->set_segment(ctxt->vcpu, &ss, VCPU_SREG_SS);
+
+        c->regs[VCPU_REGS_RCX] = c->eip;
+        if (is_long_mode(ctxt->vcpu)) {
+#ifdef CONFIG_X86_64
+                c->regs[VCPU_REGS_R11] = ctxt->eflags & ~EFLG_RF;
+
+                kvm_x86_ops->get_msr(ctxt->vcpu,
+                        ctxt->mode == X86EMUL_MODE_PROT64 ?
+                        MSR_LSTAR : MSR_CSTAR, &msr_data);
+                c->eip = msr_data;
+
+                kvm_x86_ops->get_msr(ctxt->vcpu, MSR_SYSCALL_MASK, &msr_data);
+                ctxt->eflags &= ~(msr_data | EFLG_RF);
+#endif
+        } else {
+                /* legacy mode */
+                kvm_x86_ops->get_msr(ctxt->vcpu, MSR_STAR, &msr_data);
+                c->eip = (u32)msr_data;
+
+                ctxt->eflags &= ~(EFLG_VM | EFLG_IF | EFLG_RF);
+        }
+
+        return 0;
+}
+
 int
 x86_emulate_insn(struct x86_emulate_ctxt *ctxt, struct x86_emulate_ops *ops)
 {
@@ -1993,7 +2072,10 @@ twobyte_insn:
                 }
                 break;
         case 0x05:              /* syscall */
-                goto cannot_emulate;
+                if (emulate_syscall(ctxt) == -1)
+                        goto cannot_emulate;
+                else
+                        goto writeback;
                 break;
         case 0x06:
                 emulate_clts(ctxt->vcpu);