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* [PATCH] target/i386/mshv: add MshvPutState for vcpu writes
@ 2026-11-09 12:39 Magnus Kulke
  2026-11-09 13:55 ` Mohamed Mediouni
                   ` (2 more replies)
  0 siblings, 3 replies; 5+ messages in thread
From: Magnus Kulke @ 2026-11-09 12:39 UTC (permalink / raw)
  To: qemu-devel
  Cc: Magnus Kulke, Magnus Kulke, Wei Liu, Doru Blânzeanu,
	Doru Blânzeanu

Currently QEMU marks a vcpu state as dirty also on merely inspectional
reads, e.g. via "info registers", which triggers a writeback to the
hypervisor. For MSRs that cause a side-effect when written to this is
undesired.

We introduce an MshvPutState flag into the writeback logic to differentiate
"runtime" from "full" and "reset" writebacks that should occur e.g. after
a migration or startup.

Discrete side-effectful MSRs are excluded in runtime writebacks. So far
we only mark one MSR as not runtime-safe, but probably more will be
added eventually.

Signed-off-by: Magnus Kulke <magnuskulke@linux.microsoft.com>
---
 accel/mshv/mshv-all.c       | 10 +++++-----
 include/system/mshv_int.h   | 13 +++++++++++--
 target/i386/mshv/mshv-cpu.c | 10 ++++++++--
 target/i386/mshv/msr.c      | 21 ++++++++++++++++++++-
 4 files changed, 44 insertions(+), 10 deletions(-)

diff --git a/accel/mshv/mshv-all.c b/accel/mshv/mshv-all.c
index d820a67b95..d64599e2e2 100644
--- a/accel/mshv/mshv-all.c
+++ b/accel/mshv/mshv-all.c
@@ -759,10 +759,10 @@ static int mshv_cpu_exec(CPUState *cpu)
 
     do {
         if (cpu->vcpu_dirty) {
-            ret = mshv_arch_store_vcpu_state(cpu);
+            ret = mshv_arch_store_vcpu_state(cpu, MSHV_PUT_RUNTIME_STATE);
             if (ret) {
-                error_report("Failed to put registers after init: %s",
-                              strerror(-ret));
+                error_report("Failed to put registers at runtime: %s",
+                             strerror(-ret));
                 ret = -1;
                 break;
             }
@@ -890,7 +890,7 @@ static void mshv_start_vcpu_thread(CPUState *cpu)
 static void do_mshv_cpu_synchronize_post_init(CPUState *cpu,
                                               run_on_cpu_data arg)
 {
-    int ret = mshv_arch_store_vcpu_state(cpu);
+    int ret = mshv_arch_store_vcpu_state(cpu, MSHV_PUT_FULL_STATE);
     if (ret < 0) {
         error_report("Failed to put registers after init: %s", strerror(-ret));
         abort();
@@ -906,7 +906,7 @@ static void mshv_cpu_synchronize_post_init(CPUState *cpu)
 
 static void mshv_cpu_synchronize_post_reset(CPUState *cpu)
 {
-    int ret = mshv_arch_store_vcpu_state(cpu);
+    int ret = mshv_arch_store_vcpu_state(cpu, MSHV_PUT_RESET_STATE);
     if (ret) {
         error_report("Failed to put registers after reset: %s",
                      strerror(-ret));
diff --git a/include/system/mshv_int.h b/include/system/mshv_int.h
index 1f77ffd152..b7b7c56443 100644
--- a/include/system/mshv_int.h
+++ b/include/system/mshv_int.h
@@ -98,6 +98,15 @@ typedef enum MshvVmExit {
     MshvVmExitDebug    = 3,
 } MshvVmExit;
 
+typedef enum MshvPutState {
+    /* state subset only touched by the VCPU itself during runtime */
+    MSHV_PUT_RUNTIME_STATE = 1,
+    /* state subset modified during VCPU reset */
+    MSHV_PUT_RESET_STATE = 2,
+    /* full state set, modified during initialization or on vmload */
+    MSHV_PUT_FULL_STATE = 3,
+} MshvPutState;
+
 void mshv_init_mmio_emu(void);
 int mshv_create_vcpu(int vm_fd, uint8_t vp_index, int *cpu_fd);
 void mshv_remove_vcpu(int vm_fd, int cpu_fd);
@@ -107,7 +116,7 @@ int mshv_set_generic_regs(const CPUState *cpu, const hv_register_assoc *assocs,
                           size_t n_regs);
 int mshv_get_generic_regs(CPUState *cpu, hv_register_assoc *assocs,
                           size_t n_regs);
-int mshv_arch_store_vcpu_state(const CPUState *cpu);
+int mshv_arch_store_vcpu_state(const CPUState *cpu, MshvPutState state);
 int mshv_arch_load_vcpu_state(CPUState *cpu);
 int mshv_arch_set_partition_msrs(const CPUState *cpu);
 int mshv_arch_set_mp_state(const CPUState *cpu);
@@ -152,7 +161,7 @@ void mshv_log_global_stop(MemoryListener *listener);
 /* msr */
 int mshv_init_msrs(const CPUState *cpu);
 int mshv_get_msrs(CPUState *cpu);
-int mshv_set_msrs(const CPUState *cpu);
+int mshv_set_msrs(const CPUState *cpu, MshvPutState put_state);
 
 /* synic */
 int mshv_get_simp(int cpu_fd, uint8_t *page);
diff --git a/target/i386/mshv/mshv-cpu.c b/target/i386/mshv/mshv-cpu.c
index 4df73bb497..72ef01a597 100644
--- a/target/i386/mshv/mshv-cpu.c
+++ b/target/i386/mshv/mshv-cpu.c
@@ -1511,7 +1511,13 @@ static int set_synic_state(const CPUState *cpu)
     return 0;
 }
 
-int mshv_arch_store_vcpu_state(const CPUState *cpu)
+/*
+ * The put_state param is passed to decide whether we need a full restoration
+ * of all registers (e.g. after a migration ) or just a subset during runtime
+ * (e.g after "info registers"). The latter should avoid writing registers that
+ * cause a side-effect.
+ */
+int mshv_arch_store_vcpu_state(const CPUState *cpu, MshvPutState put_state)
 {
     int ret;
 
@@ -1541,7 +1547,7 @@ int mshv_arch_store_vcpu_state(const CPUState *cpu)
         return ret;
     }
 
-    ret = mshv_set_msrs(cpu);
+    ret = mshv_set_msrs(cpu, put_state);
     if (ret < 0) {
         return ret;
     }
diff --git a/target/i386/mshv/msr.c b/target/i386/mshv/msr.c
index 96bd552916..a06844cc99 100644
--- a/target/i386/mshv/msr.c
+++ b/target/i386/mshv/msr.c
@@ -324,6 +324,20 @@ static bool msr_supported(uint32_t name)
     return true;
 }
 
+static bool msr_is_runtime_safe(uint32_t name)
+{
+    switch (name) {
+    /*
+     * HV_X64_REGISTER_TSC will invalidate the guest's tsc_page if written to
+     * while the guest's timer is not frozen.
+     */
+    case HV_X64_REGISTER_TSC:
+        return false;
+    default:
+        return true;
+    }
+}
+
 int mshv_get_msrs(CPUState *cpu)
 {
     int ret = 0;
@@ -406,13 +420,14 @@ static void load_from_env(const CPUState *cpu, struct hv_register_assoc *assocs,
     load_from_env_mtrr_phys(cpu, &assocs[mtrr_offset], MSHV_MTRR_MSR_COUNT);
 }
 
-int mshv_set_msrs(const CPUState *cpu)
+int mshv_set_msrs(const CPUState *cpu, MshvPutState put_state)
 {
     size_t n_assocs = MSHV_MSR_TOTAL_COUNT;
     struct hv_register_assoc assocs[MSHV_MSR_TOTAL_COUNT] = { 0 };
     int ret;
     size_t i, j;
     X86CPU *x86cpu = X86_CPU(cpu);
+    bool runtime = (put_state <= MSHV_PUT_RUNTIME_STATE);
 
     load_from_env(cpu, assocs, n_assocs);
 
@@ -432,6 +447,10 @@ int mshv_set_msrs(const CPUState *cpu)
             continue;
         }
 
+        if (runtime && !msr_is_runtime_safe(name)) {
+            continue;
+        }
+
         if (j != i) {
             assocs[j] = assocs[i];
         }
-- 
2.34.1



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2026-11-09 12:39 [PATCH] target/i386/mshv: add MshvPutState for vcpu writes Magnus Kulke
2026-11-09 13:55 ` Mohamed Mediouni
2026-11-09 20:34 ` Wei Liu
2026-11-11 19:41   ` Magnus Kulke
2026-11-13 12:17 ` Doru Blânzeanu

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