On Thu, May 01, 2025 at 02:36:26AM +0000, Michael Kelley wrote:

From: Peter Zijlstra <peterz@infradead.org> Sent: Wednesday, April 30, 2025 4:08 AM

quoted

@@ -528,8 +546,8 @@ void __init hyperv_init(void)
 	if (hv_isolation_type_tdx() && !ms_hyperv.paravisor_present)
 		goto skip_hypercall_pg_init;

-	hv_hypercall_pg = __vmalloc_node_range(PAGE_SIZE, 1, VMALLOC_START,
-			VMALLOC_END, GFP_KERNEL, PAGE_KERNEL_ROX,
+	hv_hypercall_pg = __vmalloc_node_range(PAGE_SIZE, 1, MODULES_VADDR,
+			MODULES_END, GFP_KERNEL, PAGE_KERNEL_ROX,

Curiosity question (which I forgot ask about in v1):  Is this change so that the
hypercall page kernel address is "close enough" for the direct call to work from
built-in code and from module code?  Or is there some other reason?

No, you nailed it. Because we want to do a direct CALL, the hypercall
page must be in the disp32 range relative to the call site. The module
address space ensures this.

quoted

 			VM_FLUSH_RESET_PERMS, NUMA_NO_NODE,
 			__builtin_return_address(0));
 	if (hv_hypercall_pg == NULL)

@@ -567,27 +585,9 @@ void __init hyperv_init(void)
 		wrmsrl(HV_X64_MSR_HYPERCALL, hypercall_msr.as_uint64);
 	}

-skip_hypercall_pg_init:
-	/*
-	 * Some versions of Hyper-V that provide IBT in guest VMs have a bug
-	 * in that there's no ENDBR64 instruction at the entry to the
-	 * hypercall page. Because hypercalls are invoked via an indirect call
-	 * to the hypercall page, all hypercall attempts fail when IBT is
-	 * enabled, and Linux panics. For such buggy versions, disable IBT.
-	 *
-	 * Fixed versions of Hyper-V always provide ENDBR64 on the hypercall
-	 * page, so if future Linux kernel versions enable IBT for 32-bit
-	 * builds, additional hypercall page hackery will be required here
-	 * to provide an ENDBR32.
-	 */
-#ifdef CONFIG_X86_KERNEL_IBT
-	if (cpu_feature_enabled(X86_FEATURE_IBT) &&
-	    *(u32 *)hv_hypercall_pg != gen_endbr()) {
-		setup_clear_cpu_cap(X86_FEATURE_IBT);
-		pr_warn("Disabling IBT because of Hyper-V bug\n");
-	}
-#endif

Nit: With this IBT code removed, the #include <asm/ibt.h> at the top
of this source code file should be removed.

Indeed so.

quoted

+	hv_set_hypercall_pg(hv_hypercall_pg);

+skip_hypercall_pg_init:
 	/*
 	 * hyperv_init() is called before LAPIC is initialized: see
 	 * apic_intr_mode_init() -> x86_platform.apic_post_init() and

The nit notwithstanding,

Reviewed-by: Michael Kelley <redacted>

Thanks!