Sean Christopherson <[email protected]> writes: > On Mon, Sep 30, 2019 at 10:57:17AM +0200, Vitaly Kuznetsov wrote: >> Sean Christopherson <[email protected]> writes: >> >> > Rework vmx_set_rflags() to avoid the extra code need to handle emulation >> > of real mode and invalid state when unrestricted guest is disabled. The >> > primary reason for doing so is to avoid the call to vmx_get_rflags(), >> > which will incur a VMREAD when RFLAGS is not already available. When >> > running nested VMs, the majority of calls to vmx_set_rflags() will occur >> > without an associated vmx_get_rflags(), i.e. when stuffing GUEST_RFLAGS >> > during transitions between vmcs01 and vmcs02. >> > >> > Note, vmx_get_rflags() guarantees RFLAGS is marked available. >> > >> > Signed-off-by: Sean Christopherson <[email protected]> >> > --- >> > arch/x86/kvm/vmx/vmx.c | 28 ++++++++++++++++++---------- >> > 1 file changed, 18 insertions(+), 10 deletions(-) >> > >> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c >> > index 83fe8b02b732..814d3e6d0264 100644 >> > --- a/arch/x86/kvm/vmx/vmx.c >> > +++ b/arch/x86/kvm/vmx/vmx.c >> > @@ -1426,18 +1426,26 @@ unsigned long vmx_get_rflags(struct kvm_vcpu *vcpu) >> > void vmx_set_rflags(struct kvm_vcpu *vcpu, unsigned long rflags) >> > { >> > struct vcpu_vmx *vmx = to_vmx(vcpu); >> > - unsigned long old_rflags = vmx_get_rflags(vcpu); >> > + unsigned long old_rflags; >> > >> > - __set_bit(VCPU_EXREG_RFLAGS, (ulong *)&vcpu->arch.regs_avail); >> > - vmx->rflags = rflags; >> > - if (vmx->rmode.vm86_active) { >> > - vmx->rmode.save_rflags = rflags; >> > - rflags |= X86_EFLAGS_IOPL | X86_EFLAGS_VM; >> > + if (enable_unrestricted_guest) { >> > + __set_bit(VCPU_EXREG_RFLAGS, (ulong *)&vcpu->arch.regs_avail); >> > + >> > + vmx->rflags = rflags; >> > + vmcs_writel(GUEST_RFLAGS, rflags); >> > + } else { >> > + old_rflags = vmx_get_rflags(vcpu); >> > + >> > + vmx->rflags = rflags; >> > + if (vmx->rmode.vm86_active) { >> > + vmx->rmode.save_rflags = rflags; >> > + rflags |= X86_EFLAGS_IOPL | X86_EFLAGS_VM; >> > + } >> > + vmcs_writel(GUEST_RFLAGS, rflags); >> > + >> > + if ((old_rflags ^ vmx->rflags) & X86_EFLAGS_VM) >> > + vmx->emulation_required = emulation_required(vcpu); >> > } >> > - vmcs_writel(GUEST_RFLAGS, rflags); >> >> We're doing vmcs_writel() in both branches so it could've stayed here, right? > > Yes, but the resulting code is a bit ugly. emulation_required() consumes > vmcs.GUEST_RFLAGS, i.e. the if statement that reads old_rflags would also > need to be outside of the else{} case. > > This isn't too bad: > > if (!enable_unrestricted_guest && > ((old_rflags ^ vmx->rflags) & X86_EFLAGS_VM)) > vmx->emulation_required = emulation_required(vcpu); > > but gcc isn't smart enough to understand old_rflags won't be used if > enable_unrestricted_guest, so old_rflags either needs to be tagged with > uninitialized_var() or explicitly initialized in the if(){} case. > > Duplicating a small amount of code felt like the lesser of two evils. >
I see, thanks for these additional details! -- Vitaly
