Re: [PATCH v9 05/18] x86/virt/tdx: Add SEAMCALL infrastructure

Dave Hansen
2023-02-13T22:39:20Z

On 2/13/23 03:59, Kai Huang wrote:
> SEAMCALL instruction causes #GP when TDX isn't BIOS enabled, and #UD
> when CPU is not in VMX operation. The current TDX_MODULE_CALL macro
> doesn't handle any of them. There's no way to check whether the CPU is
> in VMX operation or not.

Really? ... *REALLY*?

Like, there's no possible way for the kernel to record whether it has
executed VMXON or not?

I think what you're saying here is that there's no architecturally
visible flag that tells you whether in spot #1 or #2 in the following code:

static int kvm_cpu_vmxon(u64 vmxon_pointer)
{
u64 msr;

cr4_set_bits(X86_CR4_VMXE);
// spot #1
asm_volatile_goto("1: vmxon %[vmxon_pointer]\n\t"
_ASM_EXTABLE(1b, %l[fault])
: : [vmxon_pointer] "m"(vmxon_pointer)
: : fault);
// spot #2

That's _maybe_ technically correct (I don't know enough about VMX
enabling to tell you). But, what I *DO* know is that it's nonsense to
say that it's impossible in the *kernel* to tell whether we're on a CPU
that's successfully executed VMXON or not.

kvm_cpu_vmxon() has two paths through it:

1. Successfully executes VMXON and leaves with X86_CR4_VMXE=1
2. Fails VMXON and leaves with X86_CR4_VMXE=0

Guess what? CR4 is rather architecturally visible. From what I can
tell, it's *ENTIRELY* plausible to assume that X86_CR4_VMXE==1 means
that VMXON has been done. Even if that's wrong, it's only a cpumask and
a cpumask_set() away from becoming plausible. Like so:

static int kvm_cpu_vmxon(u64 vmxon_pointer)
{
u64 msr;

cr4_set_bits(X86_CR4_VMXE);

asm_volatile_goto("1: vmxon %[vmxon_pointer]\n\t"
_ASM_EXTABLE(1b, %l[fault])
: : [vmxon_pointer] "m"(vmxon_pointer)
: : fault);
// set cpumask bit here
return 0;

fault:
// clear cpu bit here
cr4_clear_bits(X86_CR4_VMXE);

return -EFAULT;
}

How many design decisions down the line in this series were predicated
on the idea that:

There's no way to check whether the CPU is
in VMX operation or not.

?