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

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Date: Mon, 22 Jan 2024 10:28:58 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: Dave Hansen <dave.hansen@...el.com>,
        Alexey Makhalov <alexey.makhalov@...adcom.com>,
        linux-kernel@...r.kernel.org, virtualization@...ts.linux.dev,
        bp@...en8.de, dave.hansen@...ux.intel.com, mingo@...hat.com,
        tglx@...utronix.de
CC: x86@...nel.org, netdev@...r.kernel.org, richardcochran@...il.com,
        linux-input@...r.kernel.org, dmitry.torokhov@...il.com,
        zackr@...are.com, linux-graphics-maintainer@...are.com,
        pv-drivers@...are.com, namit@...are.com, timothym@...are.com,
        akaher@...are.com, jsipek@...are.com, dri-devel@...ts.freedesktop.org,
        daniel@...ll.ch, airlied@...il.com, tzimmermann@...e.de,
        mripard@...nel.org, maarten.lankhorst@...ux.intel.com,
        horms@...nel.org, kirill.shutemov@...ux.intel.com
Subject: Re: [PATCH v6 7/7] x86/vmware: Add TDX hypercall support

On January 22, 2024 8:32:22 AM PST, Dave Hansen <dave.hansen@...el.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@...ux.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.