Re: [PATCH v1 05/11] x86/tdx: Add __tdx_module_call() and __tdx_hypercall() helper functions

From: "Kuppuswamy, Sathyanarayanan"  <sathyanarayanan.kuppuswamy@linux.intel.com>
To: Borislav Petkov <bp@alien8.de>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Andy Lutomirski <luto@kernel.org>, Peter H Anvin <hpa@zytor.com>,
	Dave Hansen <dave.hansen@intel.com>,
	Tony Luck <tony.luck@intel.com>,
	Dan Williams <dan.j.williams@intel.com>,
	Andi Kleen <ak@linux.intel.com>,
	Kirill Shutemov <kirill.shutemov@linux.intel.com>,
	Kuppuswamy Sathyanarayanan <knsathya@kernel.org>,
	Sean Christopherson <seanjc@google.com>,
	linux-kernel@vger.kernel.org, x86@kernel.org
Subject: Re: [PATCH v1 05/11] x86/tdx: Add __tdx_module_call() and __tdx_hypercall() helper functions
Date: Mon, 14 Jun 2021 14:37:32 -0700	[thread overview]
Message-ID: <ba91c178-406f-834c-e49c-06fa1d953971@linux.intel.com> (raw)
In-Reply-To: <YMe34ptb8CCV7Vg9@zn.tnic>

On 6/14/21 1:11 PM, Borislav Petkov wrote:
> On Mon, Jun 14, 2021 at 12:45:45PM -0700, Kuppuswamy, Sathyanarayanan wrote:
>> May be I should define a macro for it and use Mov to keep it uniform
>> with other register updates.
> 
> Macro?
> 
> There's the, well, *MOV* instruction, if you insist on keeping it
> uniform. But this is not about keeping it uniform - it is about having
> the code as clear as understandable as possible:
> 
> 
> 	/* Set RAX to TDCALL leaf function 0 */
> 	xor %eax, %eax
> 
> Plain and simple and clear why the XORing is done.

Ok. I will fix the comment.

>> With the trace support, they should be able to see the flow before making
>> the tdx_*_call(). That should be enough clue for debug right?
> 
> Are you expecting all those cloud users to trace their guests just to
> figure that out? I'm sceptical they will...
> 
> Rather, I'd try to allocate a special error value that
> do_tdx_hypercall() returns in %eax and then have the wrapper which will
> puts %r10 on the stack, check that error value and panic with a nice
> error message.
> 

I will add r10 to struct tdx_hypercall_output and return it to callers to
check it.

> 
> Btw, where is that function used? Gurgling, it shows it in some MMIO
> patch, I'm guessing that's still coming.
> 
> As to how to do it properly, you pass in
> 
> 	struct tdx_hypercall_output *out
> 
> as a function parameter and caller can pick out whatever it wants from
> that struct.

It looks like it is used only in MMIO use case now. I think we don't need
it anymore. I will remove it.

> 
> Thx.
> 

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer