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Subject: Re: [RFC v2 14/32] x86/tdx: Handle port I/O
To:     Dave Hansen <dave.hansen@intel.com>,
        Dan Williams <dan.j.williams@intel.com>
Cc:     Andi Kleen <ak@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
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        Tony Luck <tony.luck@intel.com>,
        Kirill Shutemov <kirill.shutemov@linux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@kernel.org>,
        Raj Ashok <ashok.raj@intel.com>,
        Sean Christopherson <seanjc@google.com>,
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From:   "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@linux.intel.com>
Message-ID: <abf24e52-3fee-eda1-4606-ea2aa43061d8@linux.intel.com>
Date:   Tue, 11 May 2021 08:08:20 -0700
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On 5/11/21 7:39 AM, Dave Hansen wrote:
> To what you do you refer by "this patch allocates this memory in ASM
> code"?  Could you point to the specific ASM code that "allocates memory"?

We use 40 bytes in stack for storing the output register values. It is in
function tdg_inl().

subq $TDVMCALL_OUTPUT_SIZE, %rsp

+SYM_FUNC_START(tdg_inl)
+	io_save_registers
+	/* Set data width to 4 bytes */
+	mov $4, %rsi
+1:
+	mov %rdx, %rcx
+	/* Set 0 in RDX to select in operation */
+	mov $0, %rdx
+	/* Set TDVMCALL function id in RDI */
+	mov $EXIT_REASON_IO_INSTRUCTION, %rdi
+	/* Set TDVMCALL type info (0 - Standard, > 0 - vendor) in R10 */
+	xor %r10, %r10
+	/* Allocate memory in stack for Output */
+	subq $TDVMCALL_OUTPUT_SIZE, %rsp
+	/* Move tdvmcall_output pointer to R9 */
+	movq %rsp, %r9
+
+	call do_tdvmcall

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer