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

Peter Zijlstra
2025-11-10T14:38:50Z

On Mon, Nov 10, 2025 at 05:08:35AM +0000, Naman Jain wrote:
> Provide an interface for Virtual Machine Monitor like OpenVMM and its
> use as OpenHCL paravisor to control VTL0 (Virtual trust Level).
> Expose devices and support IOCTLs for features like VTL creation,
> VTL0 memory management, context switch, making hypercalls,
> mapping VTL0 address space to VTL2 userspace, getting new VMBus
> messages and channel events in VTL2 etc.

> diff --git a/arch/x86/hyperv/hv_vtl.c b/arch/x86/hyperv/hv_vtl.c
> index 042e8712d8de..dba27e1bcc10 100644
> --- a/arch/x86/hyperv/hv_vtl.c
> +++ b/arch/x86/hyperv/hv_vtl.c
> @@ -249,3 +253,42 @@ int __init hv_vtl_early_init(void)
>
> return 0;
> }
> +
> +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);

> --- /dev/null
> +++ b/arch/x86/hyperv/mshv_vtl_asm.S
> @@ -0,0 +1,98 @@
> +/* SPDX-License-Identifier: GPL-2.0
> + *
> + * Assembly level code for mshv_vtl VTL transition
> + *
> + * Copyright (c) 2025, Microsoft Corporation.
> + *
> + * Author:
> + * Naman Jain <namjain@microsoft.com>
> + */
> +
> +#include <linux/linkage.h>
> +#include <asm/asm.h>
> +#include <asm/asm-offsets.h>
> +#include <asm/frame.h>
> +#include "mshv-asm-offsets.h"
> +
> + .text
> + .section .noinstr.text, "ax"
> +/*
> + * void __mshv_vtl_return_call(struct mshv_vtl_cpu_context *vtl0)

Can we please get a few words on the magical context here? Like no NMIs
and #DB traps and the like. Because if any of them were possible this
code would be horribly broken.

> + */
> +SYM_FUNC_START(__mshv_vtl_return_call)
> + /* Push callee save registers */
> + pushq %rbp
> + mov %rsp, %rbp
> + pushq %r12
> + pushq %r13
> + pushq %r14
> + pushq %r15
> + pushq %rbx
> +
> + /* register switch to VTL0 clobbers all registers except rax/rcx */
> + mov %_ASM_ARG1, %rax
> +
> + /* grab rbx/rbp/rsi/rdi/r8-r15 */
> + mov MSHV_VTL_CPU_CONTEXT_rbx(%rax), %rbx
> + mov MSHV_VTL_CPU_CONTEXT_rbp(%rax), %rbp
> + mov MSHV_VTL_CPU_CONTEXT_rsi(%rax), %rsi
> + mov MSHV_VTL_CPU_CONTEXT_rdi(%rax), %rdi
> + mov MSHV_VTL_CPU_CONTEXT_r8(%rax), %r8
> + mov MSHV_VTL_CPU_CONTEXT_r9(%rax), %r9
> + mov MSHV_VTL_CPU_CONTEXT_r10(%rax), %r10
> + mov MSHV_VTL_CPU_CONTEXT_r11(%rax), %r11
> + mov MSHV_VTL_CPU_CONTEXT_r12(%rax), %r12
> + mov MSHV_VTL_CPU_CONTEXT_r13(%rax), %r13
> + mov MSHV_VTL_CPU_CONTEXT_r14(%rax), %r14
> + mov MSHV_VTL_CPU_CONTEXT_r15(%rax), %r15
> +
> + mov MSHV_VTL_CPU_CONTEXT_cr2(%rax), %rdx
> + mov %rdx, %cr2
> + mov MSHV_VTL_CPU_CONTEXT_rdx(%rax), %rdx
> +
> + /* stash host registers on stack */
> + pushq %rax
> + pushq %rcx
> +
> + xor %ecx, %ecx
> +
> + /* make a hypercall to switch VTL */
> + call mshv_vtl_return_hypercall

Yuck!

This seems to build for me.

---
--- 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
--- 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 */