KVM: Don't pre-reserve xarray entries when storing empty/NULL attributesgrafted

Skip the xarray reservation loop when clearing all memory attributes, as
storing NULL only erases the entry and never needs to allocate, so no
reservation (and no cleanup of a failed one) is required in that case.

Suggested-by: Sean Christopherson <seanjc@google.com>
Cc: David Ballesteros <davimaba.v@proton.me>
Signed-off-by: Zeng Chi <zengchi@kylinos.cn>
Link: https://patch.msgid.link/20260921102442.1232375-1-zeng_chi911@163.com
[sean: split to separate patch]
Signed-off-by: Sean Christopherson <seanjc@google.com>

1 files changed, 125 insertions, 0 deletions

diff --git a/arch/x86/entry/entry.S b/arch/x86/entry/entry.S
new file mode 100644
index 000000000..2bc217bb5
--- /dev/null
+++ b/arch/x86/entry/entry.S
@@ -0,0 +1,125 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Common place for both 32- and 64-bit entry routines.
+ */
+
+#include <linux/export.h>
+#include <linux/kvm_types.h>
+#include <linux/linkage.h>
+#include <linux/objtool.h>
+#include <asm/msr-index.h>
+#include <asm/unwind_hints.h>
+#include <asm/segment.h>
+#include <asm/cache.h>
+#include <asm/cpufeatures.h>
+#include <asm/nospec-branch.h>
+
+#include "calling.h"
+
+.pushsection .noinstr.text, "ax"
+
+/* Clobbers AX, CX, DX */
+SYM_FUNC_START(write_ibpb)
+	ANNOTATE_NOENDBR
+	movl	$MSR_IA32_PRED_CMD, %ecx
+	movl	_ASM_RIP(x86_pred_cmd), %eax
+	xorl	%edx, %edx
+	wrmsr
+
+	/* Make sure IBPB clears return stack preductions too. */
+	FILL_RETURN_BUFFER %rax, RSB_CLEAR_LOOPS, X86_BUG_IBPB_NO_RET
+	RET
+SYM_FUNC_END(write_ibpb)
+EXPORT_SYMBOL_FOR_KVM(write_ibpb);
+
+SYM_FUNC_START(__WARN_trap)
+	ANNOTATE_NOENDBR
+	ANNOTATE_REACHABLE
+	ud1 (%edx), %_ASM_ARG1
+	RET
+SYM_FUNC_END(__WARN_trap)
+EXPORT_SYMBOL(__WARN_trap)
+
+.popsection
+
+/*
+ * Define the VERW operand that is disguised as entry code so that
+ * it can be referenced with KPTI enabled. This ensures VERW can be
+ * used late in exit-to-user path after page tables are switched.
+ */
+.pushsection .entry.text, "ax"
+
+.align L1_CACHE_BYTES, 0xcc
+SYM_CODE_START_NOALIGN(x86_verw_sel)
+	UNWIND_HINT_UNDEFINED
+	ANNOTATE_NOENDBR
+	.word __KERNEL_DS
+.align L1_CACHE_BYTES, 0xcc
+SYM_CODE_END(x86_verw_sel);
+EXPORT_SYMBOL_FOR_KVM(x86_verw_sel);
+
+.popsection
+
+THUNK warn_thunk_thunk, __warn_thunk
+
+/*
+ * Clang's implementation of TLS stack cookies requires the variable in
+ * question to be a TLS variable. If the variable happens to be defined as an
+ * ordinary variable with external linkage in the same compilation unit (which
+ * amounts to the whole of vmlinux with LTO enabled), Clang will drop the
+ * segment register prefix from the references, resulting in broken code. Work
+ * around this by avoiding the symbol used in -mstack-protector-guard-symbol=
+ * entirely in the C code, and use an alias emitted by the linker script
+ * instead.
+ */
+#if defined(CONFIG_STACKPROTECTOR) && defined(CONFIG_SMP)
+EXPORT_SYMBOL(__ref_stack_chk_guard);
+#endif
+
+#if IS_ENABLED(CONFIG_KVM_INTEL)
+.macro IDT_DO_EVENT_IRQOFF call_insn call_target
+	/*
+	 * Unconditionally create a stack frame, getting the correct RSP on the
+	 * stack (for x86-64) would take two instructions anyways, and RBP can
+	 * be used to restore RSP to make objtool happy (see below).
+	 */
+	push %_ASM_BP
+	mov %_ASM_SP, %_ASM_BP
+
+#ifdef CONFIG_X86_64
+	/*
+	 * Align RSP to a 16-byte boundary (to emulate CPU behavior) before
+	 * creating the synthetic interrupt stack frame for the IRQ/NMI.
+	 */
+	and  $-16, %rsp
+	push $__KERNEL_DS
+	push %rbp
+#endif
+	pushf
+	push $__KERNEL_CS
+	\call_insn \call_target
+
+	/*
+	 * "Restore" RSP from RBP, even though IRET has already unwound RSP to
+	 * the correct value.  objtool doesn't know the callee will IRET and,
+	 * without the explicit restore, thinks the stack is getting walloped.
+	 * Using an unwind hint is problematic due to x86-64's dynamic alignment.
+	 */
+	leave
+	RET
+.endm
+
+#ifndef CONFIG_X86_64
+.pushsection .text, "ax"
+SYM_FUNC_START(idt_do_interrupt_irqoff)
+	IDT_DO_EVENT_IRQOFF CALL_NOSPEC _ASM_ARG1
+SYM_FUNC_END(idt_do_interrupt_irqoff)
+.popsection
+#endif
+
+.pushsection .noinstr.text, "ax"
+SYM_FUNC_START(idt_do_nmi_irqoff)
+	IDT_DO_EVENT_IRQOFF call asm_exc_nmi_kvm_vmx
+SYM_FUNC_END(idt_do_nmi_irqoff)
+.popsection
+#endif