linux-kernel - Re: [RFC PATCH v5 083/104] KVM: x86: Split core of hypercall emulation to helper function

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Date:   Wed, 23 Mar 2022 10:53:49 -0700
From:   Isaku Yamahata <isaku.yamahata@...il.com>
To:     Sagi Shahar <sagis@...gle.com>
Cc:     "Yamahata, Isaku" <isaku.yamahata@...el.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, isaku.yamahata@...il.com,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Erdem Aktas <erdemaktas@...gle.com>,
        Connor Kuehl <ckuehl@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [RFC PATCH v5 083/104] KVM: x86: Split core of hypercall
 emulation to helper function

On Mon, Mar 21, 2022 at 11:32:21AM -0700,
Sagi Shahar <sagis@...gle.com> wrote:

> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 314ae43e07bf..9acb33a17445 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -9090,26 +9090,15 @@ static int complete_hypercall_exit(struct kvm_vcpu *vcpu)
> >         return kvm_skip_emulated_instruction(vcpu);
> >  }
> >
> > -int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
> > +unsigned long __kvm_emulate_hypercall(struct kvm_vcpu *vcpu, unsigned long nr,
> > +                                     unsigned long a0, unsigned long a1,
> > +                                     unsigned long a2, unsigned long a3,
> > +                                     int op_64_bit)
> >  {
> > -       unsigned long nr, a0, a1, a2, a3, ret;
> > -       int op_64_bit;
> > -
> > -       if (kvm_xen_hypercall_enabled(vcpu->kvm))
> > -               return kvm_xen_hypercall(vcpu);
> > -
> > -       if (kvm_hv_hypercall_enabled(vcpu))
> > -               return kvm_hv_hypercall(vcpu);
> > -
> > -       nr = kvm_rax_read(vcpu);
> > -       a0 = kvm_rbx_read(vcpu);
> > -       a1 = kvm_rcx_read(vcpu);
> > -       a2 = kvm_rdx_read(vcpu);
> > -       a3 = kvm_rsi_read(vcpu);
> > +       unsigned long ret;
> >
> >         trace_kvm_hypercall(nr, a0, a1, a2, a3);
> >
> > -       op_64_bit = is_64_bit_hypercall(vcpu);
> >         if (!op_64_bit) {
> >                 nr &= 0xFFFFFFFF;
> >                 a0 &= 0xFFFFFFFF;
> > @@ -9118,11 +9107,6 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
> >                 a3 &= 0xFFFFFFFF;
> >         }
> >
> > -       if (static_call(kvm_x86_get_cpl)(vcpu) != 0) {
> > -               ret = -KVM_EPERM;
> > -               goto out;
> > -       }
> > -
> >         ret = -KVM_ENOSYS;
> >
> >         switch (nr) {
> > @@ -9181,6 +9165,34 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
> >                 ret = -KVM_ENOSYS;
> >                 break;
> >         }
> > +       return ret;
> > +}
> > +EXPORT_SYMBOL_GPL(__kvm_emulate_hypercall);
> > +
> > +int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
> > +{
> > +       unsigned long nr, a0, a1, a2, a3, ret;
> > +       int op_64_bit;
> > +
> > +       if (kvm_xen_hypercall_enabled(vcpu->kvm))
> > +               return kvm_xen_hypercall(vcpu);
> > +
> > +       if (kvm_hv_hypercall_enabled(vcpu))
> > +               return kvm_hv_hypercall(vcpu);
> > +
> > +       nr = kvm_rax_read(vcpu);
> > +       a0 = kvm_rbx_read(vcpu);
> > +       a1 = kvm_rcx_read(vcpu);
> > +       a2 = kvm_rdx_read(vcpu);
> > +       a3 = kvm_rsi_read(vcpu);
> > +       op_64_bit = is_64_bit_mode(vcpu);
> 
> I think this should be "op_64_bit = is_64_bit_hypercall(vcpu);"
> is_64_bit_mode was replaced with is_64_bit_hypercall to support
> protected guests here:
> https://lore.kernel.org/all/87cztf8h43.fsf@vitty.brq.redhat.com/T/
> 
> Without it, op_64_bit will be set to 0 for TD VMs which will cause the
> upper 32 bit of the registers to be cleared in __kvm_emulate_hypercall

Oops, thanks for pointing it out.  I'll fix it up with next respin.
-- 
Isaku Yamahata <isaku.yamahata@...il.com>