[PATCH v5 05/13] KVM: selftests: Reuse GPR switching logic for nVMX
From: Yosry Ahmed
Date: Tue Jul 28 2026 - 13:55:45 EST
Reuse the GPR switching logic for nVMX by defining VMX_SWITCH_GPRS_ASM,
which is essentially the same as SVM_SWITCH_GPRS_ASM but also switches
RAX and doesn't switch RFLAGS, replacing the push/pop of a subset of the
registers.
The long clobber list of registers is no longer needed as registers are
saved and restored appropriately (and not clobbered by L2).
Define VMX_SWITCH_GPRS_ASM before including evmcs.h, such that it can be
used by evmcs_vmlaunch() and evmcs_vmresume().
This replaces the apparently thread-safe push/pop sequence with the
global GPR switching logic used by SVM, which isn't thread-safe at all.
However this is still an improvement because:
- The VMX logic is half-baked and prompts the UCALL clobber hack as it
doesn't properly save/restore everything. Reusing the GPR switching
logic used by SVM allows for dropping that hack.
- Hitting a problem due to half-baked GPR save/restore logic is arguably
more likely than thread-safety. Evidently, adding more involved stress
tests fails on VMX with the existing push/pop sequence. OTOH, there
are no known failures on SVM due to lack of thread-safety fo
save/restore. Only one test currently uses more than one vCPU with
nested (the memstress test).
The logical next step is to move the guest_regs to be per-vCPU,
making it thread-safe for both VMX and SVM in a proper way.
Assisted-by: Gemini:gemini-3.1-pro
Signed-off-by: Yosry Ahmed <yosry@xxxxxxxxxx>
---
.../testing/selftests/kvm/include/x86/evmcs.h | 46 +++++--------
tools/testing/selftests/kvm/include/x86/vmx.h | 69 +++++++++----------
2 files changed, 49 insertions(+), 66 deletions(-)
diff --git a/tools/testing/selftests/kvm/include/x86/evmcs.h b/tools/testing/selftests/kvm/include/x86/evmcs.h
index be79bda024bf1..82a8ea6b661f0 100644
--- a/tools/testing/selftests/kvm/include/x86/evmcs.h
+++ b/tools/testing/selftests/kvm/include/x86/evmcs.h
@@ -1207,30 +1207,23 @@ static inline int evmcs_vmlaunch(void)
current_evmcs->hv_clean_fields = 0;
- __asm__ __volatile__("push %%rbp;"
- "push %%rcx;"
- "push %%rdx;"
- "push %%rsi;"
- "push %%rdi;"
- "push $0;"
+ __asm__ __volatile__("push $0;"
"mov %%rsp, (%[host_rsp]);"
"lea 1f(%%rip), %%rax;"
"mov %%rax, (%[host_rip]);"
+ VMX_SWITCH_GPRS_ASM
"vmlaunch;"
"incq (%%rsp);"
- "1: pop %%rax;"
- "pop %%rdi;"
- "pop %%rsi;"
- "pop %%rdx;"
- "pop %%rcx;"
- "pop %%rbp;"
+ "1: ;"
+ VMX_SWITCH_GPRS_ASM
+ "pop %%rax;"
: [ret]"=&a"(ret)
: [host_rsp]"r"
((u64)¤t_evmcs->host_rsp),
[host_rip]"r"
- ((u64)¤t_evmcs->host_rip)
- : "memory", "cc", "rbx", "r8", "r9", "r10",
- "r11", "r12", "r13", "r14", "r15");
+ ((u64)¤t_evmcs->host_rip),
+ GUEST_REGS_OFFSETS
+ : "memory", "cc");
return ret;
}
@@ -1246,30 +1239,23 @@ static inline int evmcs_vmresume(void)
/* HOST_RSP */
current_evmcs->hv_clean_fields &= ~HV_VMX_ENLIGHTENED_CLEAN_FIELD_HOST_POINTER;
- __asm__ __volatile__("push %%rbp;"
- "push %%rcx;"
- "push %%rdx;"
- "push %%rsi;"
- "push %%rdi;"
- "push $0;"
+ __asm__ __volatile__("push $0;"
"mov %%rsp, (%[host_rsp]);"
"lea 1f(%%rip), %%rax;"
"mov %%rax, (%[host_rip]);"
+ VMX_SWITCH_GPRS_ASM
"vmresume;"
"incq (%%rsp);"
- "1: pop %%rax;"
- "pop %%rdi;"
- "pop %%rsi;"
- "pop %%rdx;"
- "pop %%rcx;"
- "pop %%rbp;"
+ "1: ;"
+ VMX_SWITCH_GPRS_ASM
+ "pop %%rax;"
: [ret]"=&a"(ret)
: [host_rsp]"r"
((u64)¤t_evmcs->host_rsp),
[host_rip]"r"
- ((u64)¤t_evmcs->host_rip)
- : "memory", "cc", "rbx", "r8", "r9", "r10",
- "r11", "r12", "r13", "r14", "r15");
+ ((u64)¤t_evmcs->host_rip),
+ GUEST_REGS_OFFSETS
+ : "memory", "cc");
return ret;
}
diff --git a/tools/testing/selftests/kvm/include/x86/vmx.h b/tools/testing/selftests/kvm/include/x86/vmx.h
index 4bcfd60e3aecb..04f5e34dea3ae 100644
--- a/tools/testing/selftests/kvm/include/x86/vmx.h
+++ b/tools/testing/selftests/kvm/include/x86/vmx.h
@@ -290,6 +290,23 @@ struct vmx_msr_entry {
u64 value;
} __attribute__ ((aligned(16)));
+#define VMX_SWITCH_GPRS_ASM \
+ GUEST_SWITCH_GPR_ASM(rax) \
+ GUEST_SWITCH_GPR_ASM(rbx) \
+ GUEST_SWITCH_GPR_ASM(rcx) \
+ GUEST_SWITCH_GPR_ASM(rdx) \
+ GUEST_SWITCH_GPR_ASM(rbp) \
+ GUEST_SWITCH_GPR_ASM(rsi) \
+ GUEST_SWITCH_GPR_ASM(rdi) \
+ GUEST_SWITCH_GPR_ASM(r8) \
+ GUEST_SWITCH_GPR_ASM(r9) \
+ GUEST_SWITCH_GPR_ASM(r10) \
+ GUEST_SWITCH_GPR_ASM(r11) \
+ GUEST_SWITCH_GPR_ASM(r12) \
+ GUEST_SWITCH_GPR_ASM(r13) \
+ GUEST_SWITCH_GPR_ASM(r14) \
+ GUEST_SWITCH_GPR_ASM(r15)
+
#include "evmcs.h"
static inline int vmxon(u64 phys)
@@ -363,9 +380,6 @@ static inline u64 vmptrstz(void)
return value;
}
-/*
- * No guest state (e.g. GPRs) is established by this vmlaunch.
- */
static inline int vmlaunch(void)
{
int ret;
@@ -373,34 +387,24 @@ static inline int vmlaunch(void)
if (enable_evmcs)
return evmcs_vmlaunch();
- __asm__ __volatile__("push %%rbp;"
- "push %%rcx;"
- "push %%rdx;"
- "push %%rsi;"
- "push %%rdi;"
- "push $0;"
+ __asm__ __volatile__("push $0;"
"vmwrite %%rsp, %[host_rsp];"
"lea 1f(%%rip), %%rax;"
"vmwrite %%rax, %[host_rip];"
+ VMX_SWITCH_GPRS_ASM
"vmlaunch;"
"incq (%%rsp);"
- "1: pop %%rax;"
- "pop %%rdi;"
- "pop %%rsi;"
- "pop %%rdx;"
- "pop %%rcx;"
- "pop %%rbp;"
+ "1: ;"
+ VMX_SWITCH_GPRS_ASM
+ "pop %%rax;"
: [ret]"=&a"(ret)
: [host_rsp]"r"((u64)HOST_RSP),
- [host_rip]"r"((u64)HOST_RIP)
- : "memory", "cc", "rbx", "r8", "r9", "r10",
- "r11", "r12", "r13", "r14", "r15");
+ [host_rip]"r"((u64)HOST_RIP),
+ GUEST_REGS_OFFSETS
+ : "memory", "cc");
return ret;
}
-/*
- * No guest state (e.g. GPRs) is established by this vmresume.
- */
static inline int vmresume(void)
{
int ret;
@@ -408,28 +412,21 @@ static inline int vmresume(void)
if (enable_evmcs)
return evmcs_vmresume();
- __asm__ __volatile__("push %%rbp;"
- "push %%rcx;"
- "push %%rdx;"
- "push %%rsi;"
- "push %%rdi;"
- "push $0;"
+ __asm__ __volatile__("push $0;"
"vmwrite %%rsp, %[host_rsp];"
"lea 1f(%%rip), %%rax;"
"vmwrite %%rax, %[host_rip];"
+ VMX_SWITCH_GPRS_ASM
"vmresume;"
"incq (%%rsp);"
- "1: pop %%rax;"
- "pop %%rdi;"
- "pop %%rsi;"
- "pop %%rdx;"
- "pop %%rcx;"
- "pop %%rbp;"
+ "1: ;"
+ VMX_SWITCH_GPRS_ASM
+ "pop %%rax;"
: [ret]"=&a"(ret)
: [host_rsp]"r"((u64)HOST_RSP),
- [host_rip]"r"((u64)HOST_RIP)
- : "memory", "cc", "rbx", "r8", "r9", "r10",
- "r11", "r12", "r13", "r14", "r15");
+ [host_rip]"r"((u64)HOST_RIP),
+ GUEST_REGS_OFFSETS
+ : "memory", "cc");
return ret;
}
--
2.55.0.487.gaf234c4eb3-goog