2017-06-27 16:20 GMT+08:00 Paolo Bonzini <pbonzini@xxxxxxxxxx>: > > > On 27/06/2017 05:41, Wanpeng Li wrote: >>> KVM emulates syscall so that it can trap 32-bit syscall on Intel processors. >> >> We have a discussion to not expose syscall/sysret to Intel 32-bit >> guest two years ago. https://lkml.org/lkml/2015/11/19/225 The >> syscall/sysret just makes sense against long mode instead of s/long mode/64-bit mode >> compatibility/legacy mode of Intel CPU. We will get a #UD in 32-bit >> guest, and syscall emulation is introduced by commit 66bb2ccd (KVM: >> x86 emulator: add syscall emulation) to handle it. So why we still >> expose syscall/sysret to Intel 32-bit guest? > > Because you didn't post v2 of that patch, I guess. :) > >>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c >>> index 87d3cb901935..0e846f0cb83b 100644 >>> --- a/arch/x86/kvm/x86.c >>> +++ b/arch/x86/kvm/x86.c >>> @@ -5313,6 +5313,8 @@ static void init_emulate_ctxt(struct kvm_vcpu *vcpu) >>> kvm_x86_ops->get_cs_db_l_bits(vcpu, &cs_db, &cs_l); >>> >>> ctxt->eflags = kvm_get_rflags(vcpu); >>> + ctxt->tf = (ctxt->eflags & X86_EFLAGS_TF) != 0; >>> + >> >> I guess this is used for "the sysret is executed the #DB is taken "as >> if" the syscall insn just completed", however, there is no sysret >> emulation, so how the #DB is taken after the sysret? > > No, it's used for instructions other than syscall and sysret: > >> + if (r == EMULATE_DONE && >> + (ctxt->tf || (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP))) >> + kvm_vcpu_do_singlestep(vcpu, &r); > > syscall (and sysret if it were emulated) overwrite ctxt->tf with the > value of TF at the end of the instruction. Other instructions don't, so > that singlestep depends on EFLAGS.TF before the instruction is executed. Why sysret is not emulated since SDM said that it can incur a #UD if not in 64-bit mode? Regards, Wanpeng Li