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

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Mon, Jan 24, 2022 at 06:01:54PM +0300, 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);
> +	if (err)
> +		return true;
> +
> +	*val = out.r11;
> +	return false;
> +}
> +
> +static bool tdx_mmio_read(int size, unsigned long addr, unsigned long *val)
> +{
> +	return tdx_mmio(size, false, addr, val);
> +}
> +
> +static bool tdx_mmio_write(int size, unsigned long addr, unsigned long *val)
> +{
> +	return tdx_mmio(size, true, addr, val);
> +}
> +
> +static int tdx_handle_mmio(struct pt_regs *regs, struct ve_info *ve)
> +{
> +	char buffer[MAX_INSN_SIZE];
> +	unsigned long *reg, val = 0;
> +	struct insn insn = {};
> +	enum mmio_type mmio;
> +	int size;
> +	bool err;
> +
> +	if (copy_from_kernel_nofault(buffer, (void *)regs->ip, MAX_INSN_SIZE))
> +		return -EFAULT;
> +
> +	if (insn_decode(&insn, buffer, MAX_INSN_SIZE, INSN_MODE_64))
> +		return -EFAULT;
> +
> +	mmio = insn_decode_mmio(&insn, &size);
> +	if (WARN_ON_ONCE(mmio == MMIO_DECODE_FAILED))
> +		return -EFAULT;
> +
> +	if (mmio != MMIO_WRITE_IMM && mmio != MMIO_MOVS) {
> +		reg = insn_get_modrm_reg_ptr(&insn, regs);
> +		if (!reg)
> +			return -EFAULT;
> +	}
> +
> +	switch (mmio) {
> +	case MMIO_WRITE:
> +		memcpy(&val, reg, size);
> +		err = tdx_mmio_write(size, ve->gpa, &val);
> +		break;

The return code conventions are still all mismatched and confusing:

- Most tdx_handle_*() handlers return bool (success == true)

- tdx_handle_mmio() returns int (success > 0)

- tdx_mmio*() helpers return bool (success == false)

I still don't see any benefit in arbitrarily mixing three different
return conventions, none of which matches the typical kernel style for
returning errors, unless the goal is to confuse the reader and invite
bugs.

There is precedent in traps.c for some handle_*() functions to return
bool (success == true), so if the goal is to align with that
semi-convention, that's ok.  But at the very least, please do it
consistently:

  - change tdx_mmio*() to return true on success;

  - change tdx_handle_mmio() to return bool, with 'len' passed as an
    argument.

Or, even better, just change them all to return 0 on success like 99+%
of error-returning kernel functions.

-- 
Josh