Re: [PATCHv2 08/29] x86/tdx: Handle in-kernel MMIO

[Thomas Gleixner <tglx@linutronix.de>]

On Mon, Jan 24 2022 at 18:01, Kirill A. Shutemov wrote:
>  
> +static bool tdx_mmio(int size, bool write, unsigned long addr,
> +		     unsigned long *val)
> +{
> +	struct tdx_hypercall_output out;
> +	u64 err;
> +
> +	err = _tdx_hypercall(EXIT_REASON_EPT_VIOLATION, size, write,
> +			     addr, *val, &out);

What's the purpose of storing *val as an argument for reads?

> +	if (err)
> +		return true;
> +
> +	*val = out.r11;
> +	return false;

Why is this writing back unconditionally for writes?

> +
>  bool tdx_get_ve_info(struct ve_info *ve)
>  {
>  	struct tdx_module_output out;
> @@ -219,6 +327,12 @@ static bool tdx_virt_exception_kernel(struct pt_regs *regs, struct ve_info *ve)
>  	case EXIT_REASON_CPUID:
>  		ret = tdx_handle_cpuid(regs);
>  		break;
> +	case EXIT_REASON_EPT_VIOLATION:
> +		ve->instr_len = tdx_handle_mmio(regs, ve);
> +		ret = ve->instr_len > 0;

I agree with Josh here. This is just wrong. Why returning the instr_len
as an error/success indicator? That's just a horrible idea simply
because the "error value" which is <= 0 is converted to a boolean return
value.

So what's wrong with doing the obvious here

	case EXIT_REASON_EPT_VIOLATION:
		return tdx_handle_mmio(regs, ve);

and have the handler function set ve->instr_length?

Also instead of having this not really helpful tdx_mmio() helper just
implement read and write seperately:

static bool tdx_mmio_read(int size, unsigned long addr, unsigned long *val)
{
	struct tdx_hypercall_output out;

	if (_tdx_hypercall(EXIT_REASON_EPT_VIOLATION, size, EPT_READ,
	   		   addr, 0, &out)
		return false;

	*val = out.r11;
	return true;
}

static bool tdx_mmio_write(int size, unsigned long addr, unsigned long val)
{
	return !!_tdx_hypercall(EXIT_REASON_EPT_VIOLATION, size, EPT_WRITE,
	   		   addr, val, NULL);
}

The return value is consistent with all the other handling functions
here, they return a boolean True for success. Which makes the main
handler consistent with the rest.

static bool tdx_handle_mmio(struct pt_regs *regs, struct ve_info *ve)
{
	char buffer[MAX_INSN_SIZE];
	unsigned long *reg, val;
	struct insn insn = {};
	int size, extend_size;
	enum mmio_type mmio;
        u8 extend_val = 0;
	bool ret;

	if (copy_from_kernel_nofault(buffer, (void *)regs->ip, MAX_INSN_SIZE))
		return false;

	if (insn_decode(&insn, buffer, MAX_INSN_SIZE, INSN_MODE_64))
		return false;

	mmio = insn_decode_mmio(&insn, &size);
	if (WARN_ON_ONCE(mmio == MMIO_DECODE_FAILED))
		return false;

	if (mmio != MMIO_WRITE_IMM && mmio != MMIO_MOVS) {
		reg = insn_get_modrm_reg_ptr(&insn, regs);
		if (!reg)
			return false;
	}

        ve->instr_length = insn.length;

	switch (mmio) {
	case MMIO_WRITE:
		memcpy(&val, reg, size);
                return tdx_mmio_write(size, ve->gpa, val);
	case MMIO_WRITE_IMM:
		val = insn.immediate.value;
                return tdx_mmio_write(size, ve->gpa, val);
	case MMIO_READ:
	case MMIO_READ_ZERO_EXTEND:
	case MMIO_READ_SIGN_EXTEND:
        	break;
	case MMIO_MOVS:
	case MMIO_DECODE_FAILED:
		return false;
	}

        /* Handle reads */
	if (!tdx_mmio_read(size, ve->gpa, &val))
		return false;

	switch (mmio) {
	case MMIO_READ:
		/* Zero-extend for 32-bit operation */
		extend_size = size == 4 ? sizeof(*reg) : 0;
                break;
	case MMIO_READ_ZERO_EXTEND:
		/* Zero extend based on operand size */
		extend_size = insn.opnd_bytes;
                break;
	case MMIO_READ_SIGN_EXTEND:
		/* Sign extend based on operand size */
		extend_size = insn.opnd_bytes;
                if (size == 1 && val & BIT(7))
                	extend_val = 0xFF;
                else if (size > 1 && val & BIT(15))
                	extend_val = 0xFF;
		break;
	default:
        	BUG();
	}

        if (extend_size)
		memset(reg, extend_val, extend_size);
        memcpy(reg, &val, size);
	return true;
}

Hmm?

Thanks,

        tglx