linux-kernel - Re: [PATCHv2 06/29] x86/tdx: Add MSR support for TDX guests

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Date:   Tue, 01 Feb 2022 22:38:05 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...el.com,
        luto@...nel.org, peterz@...radead.org
Cc:     sathyanarayanan.kuppuswamy@...ux.intel.com, aarcange@...hat.com,
        ak@...ux.intel.com, dan.j.williams@...el.com, david@...hat.com,
        hpa@...or.com, jgross@...e.com, jmattson@...gle.com,
        joro@...tes.org, jpoimboe@...hat.com, knsathya@...nel.org,
        pbonzini@...hat.com, sdeep@...are.com, seanjc@...gle.com,
        tony.luck@...el.com, vkuznets@...hat.com, wanpengli@...cent.com,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCHv2 06/29] x86/tdx: Add MSR support for TDX guests

On Mon, Jan 24 2022 at 18:01, Kirill A. Shutemov wrote:
> +static bool tdx_read_msr(unsigned int msr, u64 *val)
> +{
> +	struct tdx_hypercall_output out;
> +
> +	/*
> +	 * Emulate the MSR read via hypercall. More info about ABI
> +	 * can be found in TDX Guest-Host-Communication Interface
> +	 * (GHCI), sec titled "TDG.VP.VMCALL<Instruction.RDMSR>".
> +	 */
> +	if (_tdx_hypercall(EXIT_REASON_MSR_READ, msr, 0, 0, 0, &out))
> +		return false;
> +
> +	*val = out.r11;
> +
> +	return true;
> +}
> +
> +static bool tdx_write_msr(unsigned int msr, unsigned int low,
> +			       unsigned int high)
> +{
> +	u64 ret;
> +
> +	/*
> +	 * Emulate the MSR write via hypercall. More info about ABI
> +	 * can be found in TDX Guest-Host-Communication Interface
> +	 * (GHCI) sec titled "TDG.VP.VMCALL<Instruction.WRMSR>".
> +	 */
> +	ret = _tdx_hypercall(EXIT_REASON_MSR_WRITE, msr, (u64)high << 32 | low,
> +			     0, 0, NULL);
> +
> +	return ret ? false : true;
> +}
> +
>  bool tdx_get_ve_info(struct ve_info *ve)
>  {
>  	struct tdx_module_output out;
> @@ -132,11 +165,22 @@ static bool tdx_virt_exception_user(struct pt_regs *regs, struct ve_info *ve)
>  static bool tdx_virt_exception_kernel(struct pt_regs *regs, struct ve_info *ve)
>  {
>  	bool ret = false;
> +	u64 val;
>  
>  	switch (ve->exit_reason) {
>  	case EXIT_REASON_HLT:
>  		ret = tdx_halt();
>  		break;
> +	case EXIT_REASON_MSR_READ:
> +		ret = tdx_read_msr(regs->cx, &val);
> +		if (ret) {
> +			regs->ax = lower_32_bits(val);
> +			regs->dx = upper_32_bits(val);
> +		}
> +		break;

Why here?

static bool tdx_read_msr(struct pt_regs *regs)
{
	struct tdx_hypercall_output out;

	/*
	 * Emulate the MSR read via hypercall. More info about ABI
	 * can be found in TDX Guest-Host-Communication Interface
	 * (GHCI), sec titled "TDG.VP.VMCALL<Instruction.RDMSR>".
	 */
	if (_tdx_hypercall(EXIT_REASON_MSR_READ, regs->cx, 0, 0, 0, &out))
		return false;

	regs->ax = lower_32_bits(out.r11);
	regs->dx = upper_32_bits(out.r11);
	return true;
}

and

static bool tdx_read_msr(struct pt_regs *regs)
{
	/*
	 * Emulate the MSR write via hypercall. More info about ABI
	 * can be found in TDX Guest-Host-Communication Interface
	 * (GHCI) sec titled "TDG.VP.VMCALL<Instruction.WRMSR>".
	 */
	return !!_tdx_hypercall(EXIT_REASON_MSR_WRITE, regs->cx,
        			(u64)regs->dx << 32 | regs->ax,
			     	0, 0, NULL);
}

Also the switch case can be simplified as the only action after 'break;'
is 'return ret':

	switch (ve->exit_reason) {
	case EXIT_REASON_HLT:
		return tdx_halt();
	case EXIT_REASON_MSR_READ:
		return tdx_read_msr(regs);
	case EXIT_REASON_MSR_WRITE:
		return tdx_write_msr(regs);
        default:
                ....

Hmm?

Thanks,

        tglx