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Subject[PATCH v10 07/11] x86/tdx: Add HLT support for TDX guest
Date
From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>

Per Guest-Host-Communication Interface (GHCI) for Intel Trust
Domain Extensions (Intel TDX) specification, sec 3.8,
TDVMCALL[Instruction.HLT] provides HLT operation. Use it to implement
halt() and safe_halt() paravirtualization calls.

The same TDX hypercall is used to handle #VE exception due to
EXIT_REASON_HLT.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
---

Changes since v9:
* None

Changes since v8:
* Moved tdx_halt() comments near variable declaration.

Changes since v7:
* Added section title to spec reference in commit log and comments.
* Added extra comments as per review suggestion.

Changes since v6:
* None

Changes since v5:
* Replaced sti with STI in commit log and comments.
* Added comments for _tdx_hypercall() usage in _tdx_halt().
* Added new helper function _tdx_halt() to contain common
code between tdx_halt() and tdx_safe_halt().
* Renamed tdg_->tdx_.
* Removed BUG_ON() and used WARN_ONCE() for HLT emulation
failure.

Changes since v4:
* Added exception for EXIT_REASON_HLT in __tdx_hypercall() to
enable interrupts using sti.

Changes since v3:
* None

arch/x86/kernel/tdcall.S | 30 ++++++++++++++++
arch/x86/kernel/tdx.c | 75 ++++++++++++++++++++++++++++++++++++++--
2 files changed, 103 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/tdcall.S b/arch/x86/kernel/tdcall.S
index c7a5b8d79552..4b1fd1dd2ab5 100644
--- a/arch/x86/kernel/tdcall.S
+++ b/arch/x86/kernel/tdcall.S
@@ -36,6 +36,9 @@
*/
#define tdcall .byte 0x66,0x0f,0x01,0xcc

+/* HLT TDVMCALL sub-function ID */
+#define EXIT_REASON_HLT 12
+
/*
* __tdx_module_call() - Helper function used by TDX guests to request
* services from the TDX module (does not include VMM services).
@@ -235,6 +238,33 @@ SYM_FUNC_START(__tdx_hypercall)

movl $TDVMCALL_EXPOSE_REGS_MASK, %ecx

+ /*
+ * For the idle loop STI needs to be called directly before
+ * the TDCALL that enters idle (EXIT_REASON_HLT case). STI
+ * enables interrupts only one instruction later. If there
+ * are any instructions between the STI and the TDCALL for
+ * HLT then an interrupt could happen in that time, but the
+ * code would go back to sleep afterwards, which can cause
+ * longer delays.
+ *
+ * This leads to significant difference in network performance
+ * benchmarks. So add a special case for EXIT_REASON_HLT to
+ * trigger STI before TDCALL. But this change is not required
+ * for all HLT cases. So use R15 register value to identify the
+ * case which needs STI. So, if R11 is EXIT_REASON_HLT and R15
+ * is 1, then call STI before TDCALL instruction. Note that R15
+ * register is not required by TDCALL ABI when triggering the
+ * hypercall for EXIT_REASON_HLT case. So use it in software to
+ * select the STI case.
+ */
+ cmpl $EXIT_REASON_HLT, %r11d
+ jne skip_sti
+ cmpl $1, %r15d
+ jne skip_sti
+ /* Set R15 register to 0, it is unused in EXIT_REASON_HLT case */
+ xor %r15, %r15
+ sti
+skip_sti:
tdcall

/* Restore output pointer to R9 */
diff --git a/arch/x86/kernel/tdx.c b/arch/x86/kernel/tdx.c
index 1ef979008fe8..c05e8824e5e0 100644
--- a/arch/x86/kernel/tdx.c
+++ b/arch/x86/kernel/tdx.c
@@ -5,6 +5,7 @@
#define pr_fmt(fmt) "tdx: " fmt

#include <asm/tdx.h>
+#include <asm/vmx.h>

/* TDX Module call Leaf IDs */
#define TDX_GET_VEINFO 3
@@ -53,6 +54,62 @@ static inline u64 _tdx_hypercall(u64 fn, u64 r12, u64 r13, u64 r14,
return out->r10;
}

+static __cpuidle void _tdx_halt(const bool irq_disabled, const bool do_sti)
+{
+ u64 ret;
+
+ /*
+ * Emulate HLT operation via hypercall. More info about ABI
+ * can be found in TDX Guest-Host-Communication Interface
+ * (GHCI), sec 3.8 TDG.VP.VMCALL<Instruction.HLT>.
+ *
+ * The VMM uses the "IRQ disabled" param to understand IRQ
+ * enabled status (RFLAGS.IF) of TD guest and determine
+ * whether or not it should schedule the halted vCPU if an
+ * IRQ becomes pending. E.g. if IRQs are disabled the VMM
+ * can keep the vCPU in virtual HLT, even if an IRQ is
+ * pending, without hanging/breaking the guest.
+ *
+ * do_sti parameter is used by __tdx_hypercall() to decide
+ * whether to call STI instruction before executing TDCALL
+ * instruction.
+ */
+ ret = _tdx_hypercall(EXIT_REASON_HLT, irq_disabled, 0, 0, do_sti, NULL);
+
+ /*
+ * Use WARN_ONCE() to report the failure. Since tdx_*halt() calls
+ * are also used in pv_ops, #VE error handler cannot be used to
+ * report the failure.
+ */
+ WARN_ONCE(ret, "HLT instruction emulation failed\n");
+}
+
+static __cpuidle void tdx_halt(void)
+{
+ /*
+ * Non safe halt is mainly used in CPU offlining and
+ * the guest will stay in halt state. So, STI
+ * instruction call is not required (set do_sti as
+ * false).
+ */
+ const bool irq_disabled = irqs_disabled();
+ const bool do_sti = false;
+
+ _tdx_halt(irq_disabled, do_sti);
+}
+
+static __cpuidle void tdx_safe_halt(void)
+{
+ /*
+ * Since STI instruction will be called in __tdx_hypercall()
+ * set irq_disabled as false.
+ */
+ const bool irq_disabled = false;
+ const bool do_sti = true;
+
+ _tdx_halt(irq_disabled, do_sti);
+}
+
bool tdx_get_ve_info(struct ve_info *ve)
{
struct tdx_module_output out;
@@ -84,8 +141,19 @@ bool tdx_get_ve_info(struct ve_info *ve)
bool tdx_handle_virtualization_exception(struct pt_regs *regs,
struct ve_info *ve)
{
- pr_warn("Unexpected #VE: %lld\n", ve->exit_reason);
- return false;
+ switch (ve->exit_reason) {
+ case EXIT_REASON_HLT:
+ tdx_halt();
+ break;
+ default:
+ pr_warn("Unexpected #VE: %lld\n", ve->exit_reason);
+ return false;
+ }
+
+ /* After successful #VE handling, move the IP */
+ regs->ip += ve->instr_len;
+
+ return true;
}

void __init tdx_early_init(void)
@@ -95,5 +163,8 @@ void __init tdx_early_init(void)

setup_force_cpu_cap(X86_FEATURE_TDX_GUEST);

+ pv_ops.irq.safe_halt = tdx_safe_halt;
+ pv_ops.irq.halt = tdx_halt;
+
pr_info("Guest initialized\n");
}
--
2.25.1
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