Sean Christopherson <sean.j.christopherson@xxxxxxxxx> writes:
> 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.
>
I see, thanks for these additional details!
--
Vitaly
