[SeaBIOS] [PATCH v2] config: Add function to check if fw_cfg exists

[Petr Berky]

 From b06589c683a7defb4853a3b810bd7e6a12abe2d6 Mon Sep 17 00:00:00 2001
From: Petr Berky <petr.berky at email.cz>
Date: Tue, 14 Mar 2017 23:32:15 +0100
Subject: [PATCH v2] config: Add function to check if fw_cfg exists

It was found qemu_get_present_cpus_count may return impossible
number of cpus because of not checking if fw_cfg exists before
using it. That  may lead to undefined behavior of emulator,
in particular Bochs that freezes.

Signed-off-by: Petr Berky <petr.berky at email.cz>
---
  src/fw/paravirt.c | 12 +++++++++++-
  src/fw/paravirt.h |  1 +
  2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/src/fw/paravirt.c b/src/fw/paravirt.c
index 707502d..dfc69d4 100644
--- a/src/fw/paravirt.c
+++ b/src/fw/paravirt.c
@@ -32,9 +32,16 @@ u32 RamSize;
  u64 RamSizeOver4G;
  // Type of emulator platform.
  int PlatformRunningOn VARFSEG;
+// cfg enabled
+int cfg_enabled = 0;
  // cfg_dma enabled
  int cfg_dma_enabled = 0;

+inline int qemu_cfg_enabled(void)
+{
+    return cfg_enabled;
+}
+
  inline int qemu_cfg_dma_enabled(void)
  {
      return cfg_dma_enabled;
@@ -392,7 +399,9 @@ u16
  qemu_get_present_cpus_count(void)
  {
      u16 smp_count = 0;
-    qemu_cfg_read_entry(&smp_count, QEMU_CFG_NB_CPUS, sizeof(smp_count));
+    if (qemu_cfg_enabled()) {
+        qemu_cfg_read_entry(&smp_count, QEMU_CFG_NB_CPUS, 
sizeof(smp_count));
+    }
      u16 cmos_cpu_count = rtc_read(CMOS_BIOS_SMP_COUNT) + 1;
      if (smp_count < cmos_cpu_count) {
          smp_count = cmos_cpu_count;
@@ -570,6 +579,7 @@ void qemu_cfg_init(void)
          if (inb(PORT_QEMU_CFG_DATA) != sig[i])
              return;

+    cfg_enabled = 1;
      dprintf(1, "Found QEMU fw_cfg\n");

      // Detect DMA interface.
diff --git a/src/fw/paravirt.h b/src/fw/paravirt.h
index 16f3d9a..a14d83e 100644
--- a/src/fw/paravirt.h
+++ b/src/fw/paravirt.h
@@ -49,6 +49,7 @@ static inline int runningOnKVM(void) {
  // QEMU_CFG_DMA ID bit
  #define QEMU_CFG_VERSION_DMA    2

+int qemu_cfg_enabled(void);
  int qemu_cfg_dma_enabled(void);
  void qemu_preinit(void);
  void qemu_platform_setup(void);
-- 
2.11.0