Re: [PATCH v4 0/6] KVM: x86: Prep KVM hypercall handling for TDX

[Sean Christopherson <seanjc@google.com>]

On Wed, Jan 15, 2025, Binbin Wu wrote:
> On 12/19/2024 10:40 AM, Sean Christopherson wrote:
> > On Wed, 27 Nov 2024 16:43:38 -0800, Sean Christopherson wrote:
> > > Effectively v4 of Binbin's series to handle hypercall exits to userspace in
> > > a generic manner, so that TDX
> > > 
> > > Binbin and Kai, this is fairly different that what we last discussed.  While
> > > sorting through Binbin's latest patch, I stumbled on what I think/hope is an
> > > approach that will make life easier for TDX.  Rather than have common code
> > > set the return value, _and_ have TDX implement a callback to do the same for
> > > user return MSRs, just use the callback for all paths.
> > > 
> > > [...]
> > Applied patch 1 to kvm-x86 fixes.  I'm going to hold off on the rest until the
> > dust settles on the SEAMCALL interfaces, e.g. in case TDX ends up marshalling
> > state into the "normal" GPRs.
> Hi Sean, Based on your suggestions in the link
> https://lore.kernel.org/kvm/Z1suNzg2Or743a7e@google.com, the v2 of "KVM: TDX:
> TDX hypercalls may exit to userspace" is planned to morph the TDG.VP.VMCALL
> with KVM hypercall to EXIT_REASON_VMCALL and marshall r10~r14 from
> vp_enter_args in struct vcpu_tdx to the appropriate x86 registers for KVM
> hypercall handling.

...

> To test TDX, I made some modifications to your patch
> "KVM: x86: Refactor __kvm_emulate_hypercall() into a macro"
> Are the following changes make sense to you?

Yes, but I think we can go a step further and effectively revert the bulk of commit
e913ef159fad ("KVM: x86: Split core of hypercall emulation to helper function"),
i.e. have ____kvm_emulate_hypercall() read the GPRs instead of passing them in
via the macro.

> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index a2198807290b..2c5df57ad799 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -10088,9 +10088,7 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
>         if (kvm_hv_hypercall_enabled(vcpu))
>                 return kvm_hv_hypercall(vcpu);
> -       return __kvm_emulate_hypercall(vcpu, rax, rbx, rcx, rdx, rsi,
> -                                      is_64_bit_hypercall(vcpu),
> -                                      kvm_x86_call(get_cpl)(vcpu),
> +       return __kvm_emulate_hypercall(vcpu, kvm_x86_call(get_cpl)(vcpu),
>                                        complete_hypercall_exit);
>  }
>  EXPORT_SYMBOL_GPL(kvm_emulate_hypercall);
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index b00ecbfef000..989bed5b48b0 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -623,19 +623,18 @@ int ____kvm_emulate_hypercall(struct kvm_vcpu *vcpu, unsigned long nr,
>                               int op_64_bit, int cpl,
>                               int (*complete_hypercall)(struct kvm_vcpu *));
> -#define __kvm_emulate_hypercall(_vcpu, nr, a0, a1, a2, a3, op_64_bit, cpl, complete_hypercall) \
> -({                                                                                             \
> -       int __ret;                                                                              \
> -                                                                                               \
> -       __ret = ____kvm_emulate_hypercall(_vcpu,                                                \
> -                                         kvm_##nr##_read(_vcpu), kvm_##a0##_read(_vcpu),       \
> -                                         kvm_##a1##_read(_vcpu), kvm_##a2##_read(_vcpu),       \
> -                                         kvm_##a3##_read(_vcpu), op_64_bit, cpl,               \
> -                                         complete_hypercall);                                  \
> -                                                                                               \
> -       if (__ret > 0)                                                                          \
> -               __ret = complete_hypercall(_vcpu);                                              \
> -       __ret;                                                                                  \
> +#define __kvm_emulate_hypercall(_vcpu, cpl, complete_hypercall)                                \
> +({                                                                                     \
> +       int __ret;                                                                      \
> +       __ret = ____kvm_emulate_hypercall(_vcpu, kvm_rax_read(_vcpu),                   \
> +                                         kvm_rbx_read(_vcpu), kvm_rcx_read(_vcpu),     \
> +                                         kvm_rdx_read(_vcpu), kvm_rsi_read(_vcpu),     \
> +                                         is_64_bit_hypercall(_vcpu), cpl,              \
> +                                         complete_hypercall);                          \
> +                                                                                       \
> +       if (__ret > 0)                                                                  \
> +               __ret = complete_hypercall(_vcpu);                                      \
> +       __ret;                                                                          \
>  })
>  int kvm_emulate_hypercall(struct kvm_vcpu *vcpu);
> 
>