Re: [PATCH v11 2/2] Drivers: hv: Introduce mshv_vtl driver

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Nov 11, 2025 at 12:25:54PM +0530, Naman Jain wrote:

> This would have been the cleanest approach. We discussed this before and
> unfortunately it didn't work. Please find the link to this discussion:
> 
> https://lore.kernel.org/all/9f8007a3-f810-4b60-8942-e721cd6a32c4@linux.microsoft.com/
> 
> To summarize above discussion, I see below compilation error with this from
> objtool. You may have CONFIG_X86_KERNEL_IBT enabled in your workspace, which
> would have masked this.

IBT isn't the problem, the thing is running objtool on vmlinux.o vs the
individual translation units. vmlinux.o will have that symbol, while
your .S file doesn't.

>   AS      arch/x86/hyperv/mshv_vtl_asm.o
> arch/x86/hyperv/mshv_vtl_asm.o: error: objtool: static_call: can't find
> static_call_key symbol: __SCK____mshv_vtl_return_hypercall

Right, and I said you had to do that ADDRESSABLE thing. So I added a
DECLARE_STATIC_CALL() and a static_call() in hv.c, compiled it so .s and
stole the bits.

And then you get something like the below. See symbol 5, that's the
entry we need.

# readelf -sW defconfig-build/arch/x86/hyperv/mshv_vtl_asm.o

Symbol table '.symtab' contains 8 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
     1: 0000000000000000     8 OBJECT  LOCAL  DEFAULT    6 __UNIQUE_ID_addressable___SCK____mshv_vtl_return_hypercall_662.0
     2: 0000000000000000     0 SECTION LOCAL  DEFAULT    4 .noinstr.text
     3: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND __SCT____mshv_vtl_return_hypercall
     4: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND __x86_return_thunk
     5: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND __SCK____mshv_vtl_return_hypercall
     6: 0000000000000010   179 FUNC    GLOBAL DEFAULT    4 __mshv_vtl_return_call
     7: 0000000000000000    16 FUNC    GLOBAL DEFAULT    4 __pfx___mshv_vtl_return_call


---
--- a/arch/x86/hyperv/hv_vtl.c
+++ b/arch/x86/hyperv/hv_vtl.c
@@ -256,20 +256,6 @@ int __init hv_vtl_early_init(void)
 
 DEFINE_STATIC_CALL_NULL(__mshv_vtl_return_hypercall, void (*)(void));
 
-noinstr void mshv_vtl_return_hypercall(void)
-{
-	asm volatile ("call " STATIC_CALL_TRAMP_STR(__mshv_vtl_return_hypercall));
-}
-
-/*
- * ASM_CALL_CONSTRAINT is intentionally not used in above asm block before making a call to
- * __mshv_vtl_return_hypercall, to avoid rbp clobbering before actual VTL return happens.
- * This however leads to objtool complain about "call without frame pointer save/setup".
- * To ignore that warning, and inform objtool about this non-standard function,
- * STACK_FRAME_NON_STANDARD_FP is used.
- */
-STACK_FRAME_NON_STANDARD_FP(mshv_vtl_return_hypercall);
-
 void mshv_vtl_return_call_init(u64 vtl_return_offset)
 {
 	static_call_update(__mshv_vtl_return_hypercall,
--- a/arch/x86/hyperv/mshv_vtl_asm.S
+++ b/arch/x86/hyperv/mshv_vtl_asm.S
@@ -9,6 +9,7 @@
  */
 
 #include <linux/linkage.h>
+#include <linux/static_call_types.h>
 #include <asm/asm.h>
 #include <asm/asm-offsets.h>
 #include <asm/frame.h>
@@ -57,7 +58,7 @@ SYM_FUNC_START(__mshv_vtl_return_call)
 	xor %ecx, %ecx
 
 	/* make a hypercall to switch VTL */
-	call mshv_vtl_return_hypercall
+	call STATIC_CALL_TRAMP_STR(__mshv_vtl_return_hypercall)
 
 	/* stash guest registers on stack, restore saved host copies */
 	pushq %rax
@@ -96,3 +97,10 @@ SYM_FUNC_START(__mshv_vtl_return_call)
 	pop %rbp
 	RET
 SYM_FUNC_END(__mshv_vtl_return_call)
+
+	.section	.discard.addressable,"aw"
+	.align 8
+	.type	__UNIQUE_ID_addressable___SCK____mshv_vtl_return_hypercall_662.0, @object
+	.size	__UNIQUE_ID_addressable___SCK____mshv_vtl_return_hypercall_662.0, 8
+__UNIQUE_ID_addressable___SCK____mshv_vtl_return_hypercall_662.0:
+	.quad	__SCK____mshv_vtl_return_hypercall
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -11,6 +11,10 @@
 #define __has_builtin(x) (0)
 #endif
 
+/* Indirect macros required for expanded argument pasting, eg. __LINE__. */
+#define ___PASTE(a,b) a##b
+#define __PASTE(a,b) ___PASTE(a,b)
+
 #ifndef __ASSEMBLY__
 
 /*
@@ -79,10 +83,6 @@ static inline void __chk_io_ptr(const vo
 # define __builtin_warning(x, y...) (1)
 #endif /* __CHECKER__ */
 
-/* Indirect macros required for expanded argument pasting, eg. __LINE__. */
-#define ___PASTE(a,b) a##b
-#define __PASTE(a,b) ___PASTE(a,b)
-
 #ifdef __KERNEL__
 
 /* Attributes */
--- a/include/linux/static_call_types.h
+++ b/include/linux/static_call_types.h
@@ -25,6 +25,8 @@
 #define STATIC_CALL_SITE_INIT 2UL	/* init section */
 #define STATIC_CALL_SITE_FLAGS 3UL
 
+#ifndef __ASSEMBLY__
+
 /*
  * The static call site table needs to be created by external tooling (objtool
  * or a compiler plugin).
@@ -100,4 +102,6 @@ struct static_call_key {
 
 #endif /* CONFIG_HAVE_STATIC_CALL */
 
+#endif /* __ASSEMBLY__ */
+
 #endif /* _STATIC_CALL_TYPES_H */