On Mon, Sep 30, 2019 at 10:57:17AM +0200, Vitaly Kuznetsov wrote:
> Sean Christopherson <sean.j.christopherson@xxxxxxxxx> 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 <sean.j.christopherson@xxxxxxxxx>
> > ---
> > 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.
> > -
> > - if ((old_rflags ^ vmx->rflags) & X86_EFLAGS_VM)
> > - vmx->emulation_required = emulation_required(vcpu);
> > }
> >
> > u32 vmx_get_interrupt_shadow(struct kvm_vcpu *vcpu)
>
> Reviewed-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
>
> --
> Vitaly
