On 11/10/2025 8:08 PM, Peter Zijlstra wrote:
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 <[email protected]>
+ */
+
+#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.
Sure, I can add below comment:
This function is marked as 'noinstr' to prevent against instrumentation
and debugging facilities. NMIs aren't a problem because the NMI handler
saves/restores CR2 specifically to guard against #PFs in NMI context
clobbering the guest state.
+ */
+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.
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/[email protected]/
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.
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
make[4]: *** [scripts/Makefile.build:430:
arch/x86/hyperv/mshv_vtl_asm.o] Error 255
make[4]: *** Deleting file 'arch/x86/hyperv/mshv_vtl_asm.o'
make[3]: *** [scripts/Makefile.build:556: arch/x86/hyperv] Error 2
Because of this compilation error, I had to implement this wrapper
function (mshv_vtl_return_hypercall) which makes this static call in C.
---
--- 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 @@
*/
<snip>
Regards,
Naman
