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

[Alexey Makhalov <alexey.makhalov@broadcom.com>]

On 1/22/24 10:28 AM, H. Peter Anvin wrote:
> On January 22, 2024 8:32:22 AM PST, Dave Hansen <dave.hansen@intel.com> 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@linux.intel.com>
>>
> 
> 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?