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

Kirill A. Shutemov
2022-02-02T13:06:11Z

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