Re: [PATCH v19 078/130] KVM: TDX: Implement TDX vcpu enter/exit path

[Sean Christopherson]

On Mon, Feb 26, 2024, isaku.yamahata@xxxxxxxxx wrote:
> +static noinstr void tdx_vcpu_enter_exit(struct vcpu_tdx *tdx)
> +{
> +	struct tdx_module_args args;
> +
> +	/*
> +	 * Avoid section mismatch with to_tdx() with KVM_VM_BUG().  The caller
> +	 * should call to_tdx().

C'mon.  I don't think it's unreasonable to expect that at least one of the many
people working on TDX would figure out why to_vmx() is __always_inline.

> +	 */
> +	struct kvm_vcpu *vcpu = &tdx->vcpu;
> +
> +	guest_state_enter_irqoff();
> +
> +	/*
> +	 * TODO: optimization:
> +	 * - Eliminate copy between args and vcpu->arch.regs.
> +	 * - copyin/copyout registers only if (tdx->tdvmvall.regs_mask != 0)
> +	 *   which means TDG.VP.VMCALL.
> +	 */
> +	args = (struct tdx_module_args) {
> +		.rcx = tdx->tdvpr_pa,
> +#define REG(reg, REG)	.reg = vcpu->arch.regs[VCPU_REGS_ ## REG]

Organizing tdx_module_args's registers by volatile vs. non-volatile is asinine.
This code should not need to exist.

> +	WARN_ON_ONCE(!kvm_rebooting &&
> +		     (tdx->exit_reason.full & TDX_SW_ERROR) == TDX_SW_ERROR);
> +
> +	guest_state_exit_irqoff();
> +}
> +
> +fastpath_t tdx_vcpu_run(struct kvm_vcpu *vcpu)
> +{
> +	struct vcpu_tdx *tdx = to_tdx(vcpu);
> +
> +	if (unlikely(!tdx->initialized))
> +		return -EINVAL;
> +	if (unlikely(vcpu->kvm->vm_bugged)) {
> +		tdx->exit_reason.full = TDX_NON_RECOVERABLE_VCPU;
> +		return EXIT_FASTPATH_NONE;
> +	}
> +
> +	trace_kvm_entry(vcpu);
> +
> +	tdx_vcpu_enter_exit(tdx);
> +
> +	vcpu->arch.regs_avail &= ~VMX_REGS_LAZY_LOAD_SET;
> +	trace_kvm_exit(vcpu, KVM_ISA_VMX);
> +
> +	return EXIT_FASTPATH_NONE;
> +}
> +
>  void tdx_load_mmu_pgd(struct kvm_vcpu *vcpu, hpa_t root_hpa, int pgd_level)
>  {
>  	WARN_ON_ONCE(root_hpa & ~PAGE_MASK);
> diff --git a/arch/x86/kvm/vmx/tdx.h b/arch/x86/kvm/vmx/tdx.h
> index d822e790e3e5..81d301fbe638 100644
> --- a/arch/x86/kvm/vmx/tdx.h
> +++ b/arch/x86/kvm/vmx/tdx.h
> @@ -27,6 +27,37 @@ struct kvm_tdx {
>  	struct page *source_page;
>  };
>  
> +union tdx_exit_reason {
> +	struct {
> +		/* 31:0 mirror the VMX Exit Reason format */

Then use "union vmx_exit_reason", having to maintain duplicate copies of the same
union is not something I want to do.

I'm honestly not even convinced that "union tdx_exit_reason" needs to exist.  I
added vmx_exit_reason because we kept having bugs where KVM would fail to strip
bits 31:16, and because nested VMX needs to stuff failed_vmentry, but I don't
see a similar need for TDX.

I would even go so far as to say the vcpu_tdx field shouldn't be exit_reason,
and instead should be "return_code" or something.  E.g. if the TDX module refuses
to run the vCPU, there's no VM-Enter and thus no VM-Exit (unless you count the
SEAMCALL itself, har har).  Ditto for #GP or #UD on the SEAMCALL (or any other
reason that generates TDX_SW_ERROR).

Ugh, I'm doubling down on that suggesting.  This:

	WARN_ON_ONCE(!kvm_rebooting &&
		     (tdx->vp_enter_ret & TDX_SW_ERROR) == TDX_SW_ERROR);

	if ((u16)tdx->exit_reason.basic == EXIT_REASON_EXCEPTION_NMI &&
	    is_nmi(tdexit_intr_info(vcpu))) {
		kvm_before_interrupt(vcpu, KVM_HANDLING_NMI);
		vmx_do_nmi_irqoff();
		kvm_after_interrupt(vcpu);
	}

is heinous.  If there's an error that leaves bits 15:0 zero, KVM will synthesize
a spurious NMI.  I don't know whether or not that can happen, but it's not
something that should even be possible in KVM, i.e. the exit reason should be
processed if and only if KVM *knows* there was a sane VM-Exit from non-root mode.

tdx_vcpu_run() has a similar issue, though it's probably benign.  If there's an
error in bits 15:0 that happens to collide with EXIT_REASON_TDCALL, weird things
will happen.

	if (tdx->exit_reason.basic == EXIT_REASON_TDCALL)
		tdx->tdvmcall.rcx = vcpu->arch.regs[VCPU_REGS_RCX];
	else
		tdx->tdvmcall.rcx = 0;

I vote for something like the below, with much more robust checking of vp_enter_ret
before it's converted to a VMX exit reason.

static __always_inline union vmx_exit_reason tdexit_exit_reason(struct kvm_vcpu *vcpu)
{
	return (u32)vcpu->vp_enter_ret;
}

diff --git a/arch/x86/kvm/vmx/tdx.h b/arch/x86/kvm/vmx/tdx.h
index af3a2b8afee8..b9b40b2eaccb 100644
--- a/arch/x86/kvm/vmx/tdx.h
+++ b/arch/x86/kvm/vmx/tdx.h
@@ -43,37 +43,6 @@ struct kvm_tdx {
        struct page *source_page;
 };
 
-union tdx_exit_reason {
-       struct {
-               /* 31:0 mirror the VMX Exit Reason format */
-               u64 basic               : 16;
-               u64 reserved16          : 1;
-               u64 reserved17          : 1;
-               u64 reserved18          : 1;
-               u64 reserved19          : 1;
-               u64 reserved20          : 1;
-               u64 reserved21          : 1;
-               u64 reserved22          : 1;
-               u64 reserved23          : 1;
-               u64 reserved24          : 1;
-               u64 reserved25          : 1;
-               u64 bus_lock_detected   : 1;
-               u64 enclave_mode        : 1;
-               u64 smi_pending_mtf     : 1;
-               u64 smi_from_vmx_root   : 1;
-               u64 reserved30          : 1;
-               u64 failed_vmentry      : 1;
-
-               /* 63:32 are TDX specific */
-               u64 details_l1          : 8;
-               u64 class               : 8;
-               u64 reserved61_48       : 14;
-               u64 non_recoverable     : 1;
-               u64 error               : 1;
-       };
-       u64 full;
-};
-
 struct vcpu_tdx {
        struct kvm_vcpu vcpu;
 
@@ -103,7 +72,8 @@ struct vcpu_tdx {
                };
                u64 rcx;
        } tdvmcall;
-       union tdx_exit_reason exit_reason;
+
+       u64 vp_enter_ret;
 
        bool initialized;