On 8/23/2025 3:11 AM, Zhao Liu wrote:
On Wed, Jul 23, 2025 at 11:22:11AM -0700, Xin Li (Intel) wrote:
Date: Wed, 23 Jul 2025 11:22:11 -0700
From: "Xin Li (Intel)" <x...@zytor.com>
Subject: [PATCH v1 1/1] target/i386: Save/restore the nested flag of an
exception
X-Mailer: git-send-email 2.50.1
Save/restore the nested flag of an exception during VM save/restore
and live migration to ensure a correct event stack level is chosen
when a nested exception is injected through FRED event delivery.
The event stack level used by FRED event delivery depends on whether
the event was a nested exception encountered during delivery of an
earlier event, because a nested exception is "regarded" as happening
on ring 0. E.g., when #PF is configured to use stack level 1 in
IA32_FRED_STKLVLS MSR:
- nested #PF will be delivered on the stack pointed by IA32_FRED_RSP1
MSR when encountered in ring 3 and ring 0.
- normal #PF will be delivered on the stack pointed by IA32_FRED_RSP0
MSR when encountered in ring 3.
- normal #PF will be delivered on the stack pointed by IA32_FRED_RSP1
MSR when encountered in ring 0.
As such Qemu needs to track if an event is a nested event during VM
context save/restore and live migration.
Signed-off-by: Xin Li (Intel) <x...@zytor.com>
---
linux-headers/asm-x86/kvm.h | 4 +++-
linux-headers/linux/kvm.h | 1 +
target/i386/cpu.c | 1 +
target/i386/cpu.h | 1 +
target/i386/kvm/kvm.c | 35 +++++++++++++++++++++++++++++++++++
target/i386/kvm/kvm_i386.h | 1 +
target/i386/machine.c | 1 +
7 files changed, 43 insertions(+), 1 deletion(-)
diff --git a/target/i386/kvm/kvm_i386.h b/target/i386/kvm/kvm_i386.h
index 5f83e8850a..7e765b6833 100644
--- a/target/i386/kvm/kvm_i386.h
+++ b/target/i386/kvm/kvm_i386.h
@@ -54,6 +54,7 @@ typedef struct KvmCpuidInfo {
bool kvm_is_vm_type_supported(int type);
bool kvm_has_adjust_clock_stable(void);
bool kvm_has_exception_payload(void);
+bool kvm_has_exception_nested_flag(void);
void kvm_synchronize_all_tsc(void);
void kvm_get_apic_state(DeviceState *d, struct kvm_lapic_state *kapic);
diff --git a/target/i386/machine.c b/target/i386/machine.c
index dd2dac1d44..a452d2c97e 100644
--- a/target/i386/machine.c
+++ b/target/i386/machine.c
@@ -458,6 +458,7 @@ static const VMStateDescription vmstate_exception_info = {
VMSTATE_UINT8(env.exception_injected, X86CPU),
VMSTATE_UINT8(env.exception_has_payload, X86CPU),
VMSTATE_UINT64(env.exception_payload, X86CPU),
+ VMSTATE_UINT8(env.exception_is_nested, X86CPU),
A new field needs to bump up the version of vmstate_exception_info, but
I'm afraid this will break backward-migration compatibility. So what
about adding a subsction? For example,
diff --git a/target/i386/machine.c b/target/i386/machine.c
index a452d2c97e4c..6ce3cb8af6a6 100644
--- a/target/i386/machine.c
+++ b/target/i386/machine.c
@@ -433,6 +433,24 @@ static bool steal_time_msr_needed(void *opaque)
return cpu->env.steal_time_msr != 0;
}
+static bool exception_nested_needed(void *opaque)
+{
+ X86CPU *cpu = opaque;
+
+ return cpu->env.exception_is_nested;
+}
+
+static const VMStateDescription vmstate_exceprtion_nested = {
+ .name = "cpu/exception_nested",
+ .version_id = 1,
+ .minimum_version_id = 1,
+ .needed = exception_nested_needed,
+ .fields = (const VMStateField[]) {
+ VMSTATE_UINT8(env.exception_is_nested, X86CPU),
+ VMSTATE_END_OF_LIST()
+ }
+};
+
static bool exception_info_needed(void *opaque)
{
X86CPU *cpu = opaque;
@@ -458,8 +476,11 @@ static const VMStateDescription vmstate_exception_info = {
VMSTATE_UINT8(env.exception_injected, X86CPU),
VMSTATE_UINT8(env.exception_has_payload, X86CPU),
VMSTATE_UINT64(env.exception_payload, X86CPU),
- VMSTATE_UINT8(env.exception_is_nested, X86CPU),
VMSTATE_END_OF_LIST()
+ },
+ .subsections = (const VMStateDescription * const []) {
+ &vmstate_exceprtion_nested,
+ NULL,
}
};
---
Yeah, looks the right way to go.
In addition, I think it's better to update header files in a seperate
patch.
Is it a protocol that the Qemu community prefers?
Otherwise, the patch is self-contained, and not big, why break it?
Thanks!
Xin
