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

[Kirill A. Shutemov]

On Tue, Feb 01, 2022 at 10:38:05PM +0100, Thomas Gleixner wrote:
> 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?

No problem with this approach on read side.

On the write side there's one important optimization (outside of the
initial TDX enabling):

https://github.com/intel/tdx/commit/2cea8becaa5a287c93266c01fc7f2a4ed53c509d

It will require rework, maybe use separate __tdx_hypercall() for the
paravirt call implementation.

-- 
 Kirill A. Shutemov