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

Thomas Gleixner
2022-02-01T21:38:09Z

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