[PATCHv4 17/30] x86/tdx: Add port I/O emulation

Kirill A. Shutemov
2022-02-24T15:58:31Z

From: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>

TDX hypervisors cannot emulate instructions directly. This includes
port I/O which is normally emulated in the hypervisor. All port I/O
instructions inside TDX trigger the #VE exception in the guest and
would be normally emulated there.

Use a hypercall to emulate port I/O. Extend the
tdx_handle_virt_exception() and add support to handle the #VE due to
port I/O instructions.

String I/O operations are not supported in TDX. Unroll them by declaring
CC_ATTR_GUEST_UNROLL_STRING_IO confidential computing attribute.

Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
arch/x86/coco/core.c | 7 +++++-
arch/x86/coco/tdx.c | 54 ++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 60 insertions(+), 1 deletion(-)

diff --git a/arch/x86/coco/core.c b/arch/x86/coco/core.c
index 9113baebbfd2..5615b75e6fc6 100644
--- a/arch/x86/coco/core.c
+++ b/arch/x86/coco/core.c
@@ -18,7 +18,12 @@ static u64 cc_mask __ro_after_init;

static bool intel_cc_platform_has(enum cc_attr attr)
{
- return false;
+ switch (attr) {
+ case CC_ATTR_GUEST_UNROLL_STRING_IO:
+ return true;
+ default:
+ return false;
+ }
}

/*
diff --git a/arch/x86/coco/tdx.c b/arch/x86/coco/tdx.c
index 15519e498679..2e342760b1d2 100644
--- a/arch/x86/coco/tdx.c
+++ b/arch/x86/coco/tdx.c
@@ -19,6 +19,12 @@
#define EPT_READ 0
#define EPT_WRITE 1

+/* See Exit Qualification for I/O Instructions in VMX documentation */
+#define VE_IS_IO_IN(e) ((e) & BIT(3))
+#define VE_GET_IO_SIZE(e) (((e) & GENMASK(2, 0)) + 1)
+#define VE_GET_PORT_NUM(e) ((e) >> 16)
+#define VE_IS_IO_STRING(e) ((e) & BIT(4))
+
static struct {
unsigned int gpa_width;
unsigned long attributes;
@@ -292,6 +298,52 @@ static bool handle_mmio(struct pt_regs *regs, struct ve_info *ve)
return true;
}

+/*
+ * Emulate I/O using hypercall.
+ *
+ * Assumes the IO instruction was using ax, which is enforced
+ * by the standard io.h macros.
+ *
+ * Return True on success or False on failure.
+ */
+static bool handle_io(struct pt_regs *regs, u32 exit_qual)
+{
+ struct tdx_hypercall_args args = {
+ .r10 = TDX_HYPERCALL_STANDARD,
+ .r11 = EXIT_REASON_IO_INSTRUCTION,
+ };
+ int size, port;
+ u64 mask;
+ bool in, ret;
+
+ if (VE_IS_IO_STRING(exit_qual))
+ return false;
+
+ in = VE_IS_IO_IN(exit_qual);
+ size = VE_GET_IO_SIZE(exit_qual);
+ port = VE_GET_PORT_NUM(exit_qual);
+ mask = GENMASK(BITS_PER_BYTE * size, 0);
+
+ args.r12 = size;
+ args.r13 = !in;
+ args.r14 = port;
+ args.r15 = in ? 0 : regs->ax;
+
+ /*
+ * Emulate the I/O read/write via hypercall. More info about
+ * ABI can be found in TDX Guest-Host-Communication Interface
+ * (GHCI) section titled "TDG.VP.VMCALL<Instruction.IO>".
+ */
+ ret = !__tdx_hypercall(&args, in ? TDX_HCALL_HAS_OUTPUT : 0);
+ if (!ret || !in)
+ return ret;
+
+ regs->ax &= ~mask;
+ regs->ax |= ret ? args.r11 & mask : UINT_MAX;
+
+ return ret;
+}
+
void tdx_get_ve_info(struct ve_info *ve)
{
struct tdx_module_output out;
@@ -347,6 +399,8 @@ static bool virt_exception_kernel(struct pt_regs *regs, struct ve_info *ve)
return handle_cpuid(regs);
case EXIT_REASON_EPT_VIOLATION:
return handle_mmio(regs, ve);
+ case EXIT_REASON_IO_INSTRUCTION:
+ return handle_io(regs, ve->exit_qual);
default:
pr_warn("Unexpected #VE: %lld\n", ve->exit_reason);
return false;
--
2.34.1