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

[H. Peter Anvin]

On January 22, 2024 4:04:33 PM PST, Alexey Makhalov <alexey.makhalov@xxxxxxxxxxxx> wrote:
>
>
>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?
>
>

TBH that explanation didn't make much sense to me...