Re: [PATCH v8 11/11] x86/tdx: Handle CPUID via #VE

["Kuppuswamy, Sathyanarayanan" <sathyanarayanan.kuppuswamy@linux.intel.com>]

On 10/6/21 1:26 PM, Josh Poimboeuf wrote:
> On Mon, Oct 04, 2021 at 07:52:05PM -0700, Kuppuswamy Sathyanarayanan wrote:
>> +static u64 tdx_handle_cpuid(struct pt_regs *regs)
>> +{
>> +	struct tdx_hypercall_output out = {0};
>> +	u64 ret;
>> +
>> +	/*
>> +	 * Emulate CPUID instruction via hypercall. More info about
>> +	 * ABI can be found in TDX Guest-Host-Communication Interface
>> +	 * (GHCI), section titled "VP.VMCALL<Instruction.CPUID>".
>> +	 */
>> +	ret = _tdx_hypercall(EXIT_REASON_CPUID, regs->ax, regs->cx, 0, 0, &out);
>> +
>> +	/*
>> +	 * As per TDX GHCI CPUID ABI, r12-r15 registers contains contents of
>> +	 * EAX, EBX, ECX, EDX registers after CPUID instruction execution.
>> +	 * So copy the register contents back to pt_regs.
>> +	 */
>> +	regs->ax = out.r12;
>> +	regs->bx = out.r13;
>> +	regs->cx = out.r14;
>> +	regs->dx = out.r15;
> 
> Does it still make sense to save the regs if _tdx_hypercall() returns an
> error?

We don't need to save it in failure case. I will add check for error
case in next version.

> 
>> +
>> +	return ret;
> 
> Also I'm wondering about error handling for all these _tdx_hypercall()
> wrapper functions which are called by the #VE handler. >
> First, there are some inconsistencies in whether and how they return the
> r10 error.

Since we have only cared about zero/non-zero return value, we did not
check for consistency. May be I can convert all handler return values
to bool.

> 
> - _tdx_halt() warns and doesn't return anything.

Since tdx_halt handler is shared with pv_ops, we can't return anything
back (so we use WARN_ON to report the error).

> 
> - tdx_read_msr_safe() and tdx_write_msr_safe() convert all errors to -EIO.

Return value does not matter. we only check for zero/non-zero value in
tdx_handle_virtualization_exception(). we have used -EIO to convey that it is
an IO error.

> 
> - tdx_handle_cpuid() returns the raw vmcall error.
> 
> Second, as far as I can tell, the #VE handler doesn't check the actual
> return code value, other than checking for non-zero.  Should it at least
> be printed in a warning?

I don't think this is required. We can use trace to check the error code
or argument details in failure case. Since we don't really use the error
value, I am planning to change the #VE handler return type to bool.

> 

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer