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

Peter Zijlstra
2025-11-11T08:14:20Z

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 */