[PATCH 5/5] target/i386/mshv: fix pio handlers clobbering device-modified registers

["Doru Blânzeanu" <dblanzeanu@linux.microsoft.com>]

When a device handler (e.g. vmport) calls cpu_synchronize_state() during
I/O port dispatch, it sets cpu->accel->dirty = true and may modify
registers directly in env. The old PIO code ignored this: it
unconditionally wrote the stale info->rax from the VM-exit intercept
message back to the hypervisor and then cleared dirty, discarding any
register changes made by the device.

Bifurcate both handlers on cpu->accel->dirty:

handle_pio_non_str:
- dirty path: update env->eip directly. For reads (IN), merge the I/O
  result into env->regs[R_EAX] (which may have been modified by the
  device) rather than info->rax. For writes (OUT), leave RAX untouched.
  Flush all registers via mshv_store_regs() and clear dirty.
- non-dirty path: write RIP and RAX via set_x64_registers hypercall as
  before.

handle_pio_str:
- dirty path: update env->eip and the appropriate index register
  (RSI for OUTS, RDI for INS) directly. Flush via mshv_store_regs()
  and clear dirty.
- non-dirty path: write the index register and RIP via
  set_x64_registers. Drop the RAX assignment that was here before;
  string I/O does not modify RAX, and set_x64_registers is hardcoded
  to write only 2 registers so the third slot was silently ignored
  anyway.

Remove the unconditional "cpu->accel->dirty = false" at the end of both
handlers. In the non-dirty fast path it was redundant (already false).
In the dirty path it was actively harmful: it told the vcpu run loop
that env was clean when it was not, losing the device's modifications.

Signed-off-by: Doru Blânzeanu <dblanzeanu@linux.microsoft.com>
---
 target/i386/mshv/mshv-cpu.c | 72 +++++++++++++++++++++++++------------
 1 file changed, 50 insertions(+), 22 deletions(-)

diff --git a/target/i386/mshv/mshv-cpu.c b/target/i386/mshv/mshv-cpu.c
index 8a59a0b40e..5e680351cd 100644
--- a/target/i386/mshv/mshv-cpu.c
+++ b/target/i386/mshv/mshv-cpu.c
@@ -1646,10 +1646,12 @@ static int handle_pio_non_str(CPUState *cpu,
     uint32_t val, eax;
     const uint32_t eax_mask =  0xffffffffu >> (32 - len * 8);
     size_t insn_len;
-    uint64_t rip, rax;
+    uint64_t rip;
     uint32_t reg_names[2];
     uint64_t reg_values[2];
     uint16_t port = info->port_number;
+    X86CPU *x86_cpu = X86_CPU(cpu);
+    CPUX86State *env = &x86_cpu->env;
 
     if (access_type == HV_X64_INTERCEPT_ACCESS_TYPE_WRITE) {
         union {
@@ -1680,21 +1682,36 @@ static int handle_pio_non_str(CPUState *cpu,
 
     /* Advance RIP and update RAX */
     rip = info->header.rip + insn_len;
-    rax = info->rax;
 
-    reg_names[0] = HV_X64_REGISTER_RIP;
-    reg_values[0] = rip;
-    reg_names[1] = HV_X64_REGISTER_RAX;
-    reg_values[1] = rax;
+    if (cpu->vcpu_dirty) {
+        env->eip = rip;
+        if (access_type != HV_X64_INTERCEPT_ACCESS_TYPE_WRITE) {
+            /*
+             * For reads, merge the I/O result into the current RAX.
+             * Use env->regs[R_EAX] as the base since a device handler
+             * (e.g. vmport) may have called cpu_synchronize_state()
+             * and modified registers.
+             */
+            eax = (((uint32_t)env->regs[R_EAX]) & ~eax_mask)
+                  | (val & eax_mask);
+            env->regs[R_EAX] = (uint64_t)eax;
+        }
+        /* Sync modified standard registers back and clear dirty. */
+        store_regs(cpu);
+        cpu->vcpu_dirty = false;
+    } else {
+        reg_names[0] = HV_X64_REGISTER_RIP;
+        reg_values[0] = rip;
+        reg_names[1] = HV_X64_REGISTER_RAX;
+        reg_values[1] = info->rax;
 
-    ret = set_x64_registers(cpu, reg_names, reg_values);
-    if (ret < 0) {
-        error_report("Failed to set x64 registers");
-        return -1;
+        ret = set_x64_registers(cpu, reg_names, reg_values);
+        if (ret < 0) {
+            error_report("Failed to set x64 registers");
+            return -1;
+        }
     }
 
-    cpu->vcpu_dirty = false;
-
     return 0;
 }
 
@@ -1810,6 +1827,7 @@ static int handle_pio_str(CPUState *cpu, hv_x64_io_port_intercept_message *info)
     bool repop = info->access_info.rep_prefix == 1;
     size_t repeat = repop ? info->rcx : 1;
     size_t insn_len = info->header.instruction_length;
+    uint64_t rip;
     bool direction_flag;
     uint32_t reg_names[3];
     uint64_t reg_values[3];
@@ -1839,18 +1857,28 @@ static int handle_pio_str(CPUState *cpu, hv_x64_io_port_intercept_message *info)
         reg_values[0] = info->rdi;
     }
 
-    reg_names[1] = HV_X64_REGISTER_RIP;
-    reg_values[1] = info->header.rip + insn_len;
-    reg_names[2] = HV_X64_REGISTER_RAX;
-    reg_values[2] = info->rax;
+    rip = info->header.rip + insn_len;
 
-    ret = set_x64_registers(cpu, reg_names, reg_values);
-    if (ret < 0) {
-        error_report("Failed to set RIP and RAX registers");
-        return -1;
-    }
+    if (cpu->vcpu_dirty) {
+        env->eip = rip;
+        if (access_type == HV_X64_INTERCEPT_ACCESS_TYPE_WRITE) {
+            env->regs[R_ESI] = info->rsi;
+        } else {
+            env->regs[R_EDI] = info->rdi;
+        }
+        /* Sync modified standard registers back and clear dirty. */
+        store_regs(cpu);
+        cpu->vcpu_dirty = false;
+    } else {
+        reg_names[1] = HV_X64_REGISTER_RIP;
+        reg_values[1] = rip;
 
-    cpu->vcpu_dirty = false;
+        ret = set_x64_registers(cpu, reg_names, reg_values);
+        if (ret < 0) {
+            error_report("Failed to set x64 registers");
+            return -1;
+        }
+    }
 
     return 0;
 }
-- 
2.53.0