[PATCHv4 09/30] x86/tdx: Add MSR support for TDX guests

Kirill A. Shutemov
2022-02-24T15:58:37Z

Use hypercall to emulate MSR read/write for the TDX platform.

There are two viable approaches for doing MSRs in a TD guest:

1. Execute the RDMSR/WRMSR instructions like most VMs and bare metal
do. Some will succeed, others will cause a #VE. All of those that
cause a #VE will be handled with a TDCALL.
2. Use paravirt infrastructure. The paravirt hook has to keep a list
of which MSRs would cause a #VE and use a TDCALL. All other MSRs
execute RDMSR/WRMSR instructions directly.

The second option can be ruled out because the list of MSRs was
challenging to maintain. That leaves option #1 as the only viable
solution for the minimal TDX support.

For performance-critical MSR writes (like TSC_DEADLINE), future patches
will replace the WRMSR/#VE sequence with the direct TDCALL.

RDMSR and WRMSR specification details can be found in
Guest-Host-Communication Interface (GHCI) for Intel Trust Domain
Extensions (Intel TDX) specification, sec titled "TDG.VP.
VMCALL<Instruction.RDMSR>" and "TDG.VP.VMCALL<Instruction.WRMSR>".

Co-developed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
arch/x86/coco/tdx.c | 42 ++++++++++++++++++++++++++++++++++++++++++
1 file changed, 42 insertions(+)

diff --git a/arch/x86/coco/tdx.c b/arch/x86/coco/tdx.c
index 0a2e6be0cdae..89992593a209 100644
--- a/arch/x86/coco/tdx.c
+++ b/arch/x86/coco/tdx.c
@@ -116,6 +116,44 @@ void __cpuidle tdx_safe_halt(void)
WARN_ONCE(1, "HLT instruction emulation failed\n");
}

+static bool read_msr(struct pt_regs *regs)
+{
+ struct tdx_hypercall_args args = {
+ .r10 = TDX_HYPERCALL_STANDARD,
+ .r11 = EXIT_REASON_MSR_READ,
+ .r12 = regs->cx,
+ };
+
+ /*
+ * Emulate the MSR read via hypercall. More info about ABI
+ * can be found in TDX Guest-Host-Communication Interface
+ * (GHCI), section titled "TDG.VP.VMCALL<Instruction.RDMSR>".
+ */
+ if (__tdx_hypercall(&args, TDX_HCALL_HAS_OUTPUT))
+ return false;
+
+ regs->ax = lower_32_bits(args.r11);
+ regs->dx = upper_32_bits(args.r11);
+ return true;
+}
+
+static bool write_msr(struct pt_regs *regs)
+{
+ struct tdx_hypercall_args args = {
+ .r10 = TDX_HYPERCALL_STANDARD,
+ .r11 = EXIT_REASON_MSR_WRITE,
+ .r12 = regs->cx,
+ .r13 = (u64)regs->dx << 32 | regs->ax,
+ };
+
+ /*
+ * Emulate the MSR write via hypercall. More info about ABI
+ * can be found in TDX Guest-Host-Communication Interface
+ * (GHCI) section titled "TDG.VP.VMCALL<Instruction.WRMSR>".
+ */
+ return !__tdx_hypercall(&args, 0);
+}
+
void tdx_get_ve_info(struct ve_info *ve)
{
struct tdx_module_output out;
@@ -158,6 +196,10 @@ static bool virt_exception_kernel(struct pt_regs *regs, struct ve_info *ve)
switch (ve->exit_reason) {
case EXIT_REASON_HLT:
return handle_halt();
+ case EXIT_REASON_MSR_READ:
+ return read_msr(regs);
+ case EXIT_REASON_MSR_WRITE:
+ return write_msr(regs);
default:
pr_warn("Unexpected #VE: %lld\n", ve->exit_reason);
return false;
--
2.34.1