[PATCH v2 11/13] x86,hyperv: Clean up hv_do_hypercall()

Peter Zijlstra
2025-04-30T11:26:47Z

What used to be a simple few instructions has turned into a giant mess
(for x86_64). Not only does it use static_branch wrong, it mixes it
with dynamic branches for no apparent reason.

Notably it uses static_branch through an out-of-line function call,
which completely defeats the purpose, since instead of a simple
JMP/NOP site, you get a CALL+RET+TEST+Jcc sequence in return, which is
absolutely idiotic.

Add to that a dynamic test of hyperv_paravisor_present, something
which is set once and never changed.

Replace all this idiocy with a single direct function call to the
right hypercall variant.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
arch/x86/hyperv/hv_init.c | 20 +++++
arch/x86/hyperv/ivm.c | 15 ++++
arch/x86/include/asm/mshyperv.h | 137 +++++++++++-----------------------------
arch/x86/kernel/cpu/mshyperv.c | 19 +++--
4 files changed, 89 insertions(+), 102 deletions(-)

--- a/arch/x86/hyperv/hv_init.c
+++ b/arch/x86/hyperv/hv_init.c
@@ -35,7 +35,27 @@
#include <linux/highmem.h>

void *hv_hypercall_pg;
+
+#ifdef CONFIG_X86_64
+u64 hv_std_hypercall(u64 control, u64 param1, u64 param2)
+{
+ u64 hv_status;
+
+ if (!hv_hypercall_pg)
+ return U64_MAX;
+
+ register u64 __r8 asm("r8") = param2;
+ asm volatile (CALL_NOSPEC
+ : "=a" (hv_status), ASM_CALL_CONSTRAINT,
+ "+c" (control), "+d" (param1), "+r" (__r8)
+ : THUNK_TARGET(hv_hypercall_pg)
+ : "cc", "memory", "r9", "r10", "r11");
+
+ return hv_status;
+}
+#else
EXPORT_SYMBOL_GPL(hv_hypercall_pg);
+#endif

union hv_ghcb * __percpu *hv_ghcb_pg;

--- a/arch/x86/hyperv/ivm.c
+++ b/arch/x86/hyperv/ivm.c
@@ -376,9 +376,23 @@ int hv_snp_boot_ap(u32 cpu, unsigned lon
return ret;
}

+u64 hv_snp_hypercall(u64 control, u64 param1, u64 param2)
+{
+ u64 hv_status;
+
+ register u64 __r8 asm("r8") = param2;
+ asm volatile("vmmcall"
+ : "=a" (hv_status), ASM_CALL_CONSTRAINT,
+ "+c" (control), "+d" (param1), "+r" (__r8)
+ : : "cc", "memory", "r9", "r10", "r11");
+
+ return hv_status;
+}
+
#else
static inline void hv_ghcb_msr_write(u64 msr, u64 value) {}
static inline void hv_ghcb_msr_read(u64 msr, u64 *value) {}
+u64 hv_snp_hypercall(u64 control, u64 param1, u64 param2) { return U64_MAX; }
#endif /* CONFIG_AMD_MEM_ENCRYPT */

#ifdef CONFIG_INTEL_TDX_GUEST
@@ -428,6 +442,7 @@ u64 hv_tdx_hypercall(u64 control, u64 pa
#else
static inline void hv_tdx_msr_write(u64 msr, u64 value) {}
static inline void hv_tdx_msr_read(u64 msr, u64 *value) {}
+u64 hv_tdx_hypercall(u64 control, u64 param1, u64 param2) { return U64_MAX; }
#endif /* CONFIG_INTEL_TDX_GUEST */

#if defined(CONFIG_AMD_MEM_ENCRYPT) || defined(CONFIG_INTEL_TDX_GUEST)
--- a/arch/x86/include/asm/mshyperv.h
+++ b/arch/x86/include/asm/mshyperv.h
@@ -6,6 +6,7 @@
#include <linux/nmi.h>
#include <linux/msi.h>
#include <linux/io.h>
+#include <linux/static_call.h>
#include <asm/nospec-branch.h>
#include <asm/paravirt.h>
#include <hyperv/hvhdk.h>
@@ -38,16 +39,21 @@ static inline unsigned char hv_get_nmi_r
return 0;
}

-#if IS_ENABLED(CONFIG_HYPERV)
-extern bool hyperv_paravisor_present;
+extern u64 hv_tdx_hypercall(u64 control, u64 param1, u64 param2);
+extern u64 hv_snp_hypercall(u64 control, u64 param1, u64 param2);
+extern u64 hv_std_hypercall(u64 control, u64 param1, u64 param2);

+#if IS_ENABLED(CONFIG_HYPERV)
extern void *hv_hypercall_pg;

extern union hv_ghcb * __percpu *hv_ghcb_pg;

bool hv_isolation_type_snp(void);
bool hv_isolation_type_tdx(void);
-u64 hv_tdx_hypercall(u64 control, u64 param1, u64 param2);
+
+#ifdef CONFIG_X86_64
+DECLARE_STATIC_CALL(hv_hypercall, hv_std_hypercall);
+#endif

/*
* DEFAULT INIT GPAT and SEGMENT LIMIT value in struct VMSA
@@ -64,37 +70,15 @@ static inline u64 hv_do_hypercall(u64 co
{
u64 input_address = input ? virt_to_phys(input) : 0;
u64 output_address = output ? virt_to_phys(output) : 0;
- u64 hv_status;

#ifdef CONFIG_X86_64
- if (hv_isolation_type_tdx() && !hyperv_paravisor_present)
- return hv_tdx_hypercall(control, input_address, output_address);
-
- if (hv_isolation_type_snp() && !hyperv_paravisor_present) {
- __asm__ __volatile__("mov %[output_address], %%r8\n"
- "vmmcall"
- : "=a" (hv_status), ASM_CALL_CONSTRAINT,
- "+c" (control), "+d" (input_address)
- : [output_address] "r" (output_address)
- : "cc", "memory", "r8", "r9", "r10", "r11");
- return hv_status;
- }
-
- if (!hv_hypercall_pg)
- return U64_MAX;
-
- __asm__ __volatile__("mov %[output_address], %%r8\n"
- CALL_NOSPEC
- : "=a" (hv_status), ASM_CALL_CONSTRAINT,
- "+c" (control), "+d" (input_address)
- : [output_address] "r" (output_address),
- THUNK_TARGET(hv_hypercall_pg)
- : "cc", "memory", "r8", "r9", "r10", "r11");
+ return static_call_mod(hv_hypercall)(control, input_address, output_address);
#else
u32 input_address_hi = upper_32_bits(input_address);
u32 input_address_lo = lower_32_bits(input_address);
u32 output_address_hi = upper_32_bits(output_address);
u32 output_address_lo = lower_32_bits(output_address);
+ u64 hv_status;

if (!hv_hypercall_pg)
return U64_MAX;
@@ -107,8 +91,8 @@ static inline u64 hv_do_hypercall(u64 co
"D"(output_address_hi), "S"(output_address_lo),
THUNK_TARGET(hv_hypercall_pg)
: "cc", "memory");
-#endif /* !x86_64 */
return hv_status;
+#endif /* !x86_64 */
}

/* Hypercall to the L0 hypervisor */
@@ -120,41 +104,23 @@ static inline u64 hv_do_nested_hypercall
/* Fast hypercall with 8 bytes of input and no output */
static inline u64 _hv_do_fast_hypercall8(u64 control, u64 input1)
{
- u64 hv_status;
-
#ifdef CONFIG_X86_64
- if (hv_isolation_type_tdx() && !hyperv_paravisor_present)
- return hv_tdx_hypercall(control, input1, 0);
-
- if (hv_isolation_type_snp() && !hyperv_paravisor_present) {
- __asm__ __volatile__(
- "vmmcall"
- : "=a" (hv_status), ASM_CALL_CONSTRAINT,
- "+c" (control), "+d" (input1)
- :: "cc", "r8", "r9", "r10", "r11");
- } else {
- __asm__ __volatile__(CALL_NOSPEC
- : "=a" (hv_status), ASM_CALL_CONSTRAINT,
- "+c" (control), "+d" (input1)
- : THUNK_TARGET(hv_hypercall_pg)
- : "cc", "r8", "r9", "r10", "r11");
- }
+ return static_call_mod(hv_hypercall)(control, input1, 0);
#else
- {
- u32 input1_hi = upper_32_bits(input1);
- u32 input1_lo = lower_32_bits(input1);
-
- __asm__ __volatile__ (CALL_NOSPEC
- : "=A"(hv_status),
- "+c"(input1_lo),
- ASM_CALL_CONSTRAINT
- : "A" (control),
- "b" (input1_hi),
- THUNK_TARGET(hv_hypercall_pg)
- : "cc", "edi", "esi");
- }
-#endif
+ u32 input1_hi = upper_32_bits(input1);
+ u32 input1_lo = lower_32_bits(input1);
+ u64 hv_status;
+
+ __asm__ __volatile__ (CALL_NOSPEC
+ : "=A"(hv_status),
+ "+c"(input1_lo),
+ ASM_CALL_CONSTRAINT
+ : "A" (control),
+ "b" (input1_hi),
+ THUNK_TARGET(hv_hypercall_pg)
+ : "cc", "edi", "esi");
return hv_status;
+#endif
}

static inline u64 hv_do_fast_hypercall8(u16 code, u64 input1)
@@ -174,45 +140,24 @@ static inline u64 hv_do_fast_nested_hype
/* Fast hypercall with 16 bytes of input */
static inline u64 _hv_do_fast_hypercall16(u64 control, u64 input1, u64 input2)
{
- u64 hv_status;
-
#ifdef CONFIG_X86_64
- if (hv_isolation_type_tdx() && !hyperv_paravisor_present)
- return hv_tdx_hypercall(control, input1, input2);
-
- if (hv_isolation_type_snp() && !hyperv_paravisor_present) {
- __asm__ __volatile__("mov %[input2], %%r8\n"
- "vmmcall"
- : "=a" (hv_status), ASM_CALL_CONSTRAINT,
- "+c" (control), "+d" (input1)
- : [input2] "r" (input2)
- : "cc", "r8", "r9", "r10", "r11");
- } else {
- __asm__ __volatile__("mov %[input2], %%r8\n"
- CALL_NOSPEC
- : "=a" (hv_status), ASM_CALL_CONSTRAINT,
- "+c" (control), "+d" (input1)
- : [input2] "r" (input2),
- THUNK_TARGET(hv_hypercall_pg)
- : "cc", "r8", "r9", "r10", "r11");
- }
+ return static_call_mod(hv_hypercall)(control, input1, input2);
#else
- {
- u32 input1_hi = upper_32_bits(input1);
- u32 input1_lo = lower_32_bits(input1);
- u32 input2_hi = upper_32_bits(input2);
- u32 input2_lo = lower_32_bits(input2);
-
- __asm__ __volatile__ (CALL_NOSPEC
- : "=A"(hv_status),
- "+c"(input1_lo), ASM_CALL_CONSTRAINT
- : "A" (control), "b" (input1_hi),
- "D"(input2_hi), "S"(input2_lo),
- THUNK_TARGET(hv_hypercall_pg)
- : "cc");
- }
-#endif
+ u32 input1_hi = upper_32_bits(input1);
+ u32 input1_lo = lower_32_bits(input1);
+ u32 input2_hi = upper_32_bits(input2);
+ u32 input2_lo = lower_32_bits(input2);
+ u64 hv_status;
+
+ __asm__ __volatile__ (CALL_NOSPEC
+ : "=A"(hv_status),
+ "+c"(input1_lo), ASM_CALL_CONSTRAINT
+ : "A" (control), "b" (input1_hi),
+ "D"(input2_hi), "S"(input2_lo),
+ THUNK_TARGET(hv_hypercall_pg)
+ : "cc");
return hv_status;
+#endif
}

static inline u64 hv_do_fast_hypercall16(u16 code, u64 input1, u64 input2)
--- a/arch/x86/kernel/cpu/mshyperv.c
+++ b/arch/x86/kernel/cpu/mshyperv.c
@@ -37,10 +37,6 @@
bool hv_nested;
struct ms_hyperv_info ms_hyperv;

-/* Used in modules via hv_do_hypercall(): see arch/x86/include/asm/mshyperv.h */
-bool hyperv_paravisor_present __ro_after_init;
-EXPORT_SYMBOL_GPL(hyperv_paravisor_present);
-
#if IS_ENABLED(CONFIG_HYPERV)
static inline unsigned int hv_get_nested_msr(unsigned int reg)
{
@@ -287,8 +283,18 @@ static void __init x86_setup_ops_for_tsc
old_restore_sched_clock_state = x86_platform.restore_sched_clock_state;
x86_platform.restore_sched_clock_state = hv_restore_sched_clock_state;
}
+
+#ifdef CONFIG_X86_64
+DEFINE_STATIC_CALL(hv_hypercall, hv_std_hypercall);
+EXPORT_STATIC_CALL_TRAMP_GPL(hv_hypercall);
+#define hypercall_update(hc) static_call_update(hv_hypercall, hc)
+#endif
#endif /* CONFIG_HYPERV */

+#ifndef hypercall_update
+#define hypercall_update(hc) (void)hc
+#endif
+
static uint32_t __init ms_hyperv_platform(void)
{
u32 eax;
@@ -483,14 +489,14 @@ static void __init ms_hyperv_init_platfo
ms_hyperv.shared_gpa_boundary =
BIT_ULL(ms_hyperv.shared_gpa_boundary_bits);

- hyperv_paravisor_present = !!ms_hyperv.paravisor_present;
-
pr_info("Hyper-V: Isolation Config: Group A 0x%x, Group B 0x%x\n",
ms_hyperv.isolation_config_a, ms_hyperv.isolation_config_b);

if (hv_get_isolation_type() == HV_ISOLATION_TYPE_SNP) {
static_branch_enable(&isolation_type_snp);
+ if (!ms_hyperv.paravisor_present)
+ hypercall_update(hv_snp_hypercall);
} else if (hv_get_isolation_type() == HV_ISOLATION_TYPE_TDX) {
static_branch_enable(&isolation_type_tdx);

@@ -498,6 +504,7 @@ static void __init ms_hyperv_init_platfo
ms_hyperv.hints &= ~HV_X64_APIC_ACCESS_RECOMMENDED;

if (!ms_hyperv.paravisor_present) {
+ hypercall_update(hv_tdx_hypercall);
/*
* Mark the Hyper-V TSC page feature as disabled
* in a TDX VM without paravisor so that the