Re: [PATCH v6 7/7] x86/vmware: Add TDX hypercall support

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On 1/22/24 10:28 AM, H. Peter Anvin wrote:
On January 22, 2024 8:32:22 AM PST, Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
On 1/9/24 00:40, Alexey Makhalov wrote:
+#ifdef CONFIG_INTEL_TDX_GUEST
+unsigned long vmware_tdx_hypercall(unsigned long cmd,
+				   struct tdx_module_args *args)
+{
+	if (!hypervisor_is_type(X86_HYPER_VMWARE))
+		return ULONG_MAX;
+
+	if (cmd & ~VMWARE_CMD_MASK) {
+		pr_warn_once("Out of range command %lx\n", cmd);
+		return ULONG_MAX;
+	}
+
+	args->r10 = VMWARE_TDX_VENDOR_LEAF;
+	args->r11 = VMWARE_TDX_HCALL_FUNC;
+	args->r12 = VMWARE_HYPERVISOR_MAGIC;
+	args->r13 = cmd;
+	args->r15 = 0; /* CPL */
+
+	__tdx_hypercall(args);
+
+	return args->r12;
+}
+EXPORT_SYMBOL_GPL(vmware_tdx_hypercall);
+#endif

This is the kind of wrapper that I was hoping for.  Thanks.

Acked-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>


I'm slightly confused by this TBH.

Why are the arguments passed in as a structure, which is modified by the wrapper to boot? This is analogous to a system call interface.

Furthermore, this is an out-of-line function; it should never be called with !X86_HYPER_VMWARE or you are introducing overhead for other hypervisors; I believe a pr_warn_once() is in order at least, just as you have for the out-of-range test.


This patch series introduces vmware_hypercall family of functions similar to kvm_hypercall. Similarity: both vmware and kvm implementations are static inline functions and both of them use __tdx_hypercall (global not exported symbol). Difference: kvm_hypercall functions are used _only_ within the kernel, but vmware_hypercall are also used by modules.
Exporting __tdx_hypercall function is an original Dave's concern.
So we ended up with exporting wrapper, not generic, but VMware specific with added checks against arbitrary use. vmware_tdx_hypercall is not designed for !X86_HYPER_VMWARE callers. But such a calls are not forbidden. Arguments in a structure is an API for __tdx_hypercall(). Input and output argument handling are done by vmware_hypercall callers, while VMware specific dress up is inside the wrapper.

Peter, do you think code comments are required to make it clear for the reader?






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