Check for pae and long mode using cpuid.  If present also read the
physical address bits.  Apply some qemu sanity checks (see below).
Record results in PhysBits and LongMode variables.  In case we are not
sure what the address space size is leave the PhysBits variable unset.

On qemu we have the problem that for historical reasons x86_64
processors advertise 40 physical address space bits by default, even in
case the host supports less than that so actually using the whole
address space will not work.

Because of that the code applies some extra sanity checks in case we
find 40 (or less) physical address space bits advertised.  Only
known-good values (which is 40 for amd processors and 36+39 for intel
processors) will be accepted as valid.

Recommendation is to use 'qemu -cpu ${name},host-phys-bits=on' to
advertise valid physical address space bits to the guest.  Some distro
builds enable this by default, and most likely the qemu default will
change in near future too.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
---
 src/fw/paravirt.h |  2 ++
 src/fw/paravirt.c | 57 +++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 59 insertions(+)

diff --git a/src/fw/paravirt.h b/src/fw/paravirt.h
index 4e2e993ba9d3..7ca3fdcc06d4 100644
--- a/src/fw/paravirt.h
+++ b/src/fw/paravirt.h
@@ -31,6 +31,8 @@ typedef struct QemuCfgDmaAccess {
 extern u32 RamSize;
 extern u64 RamSizeOver4G;
 extern int PlatformRunningOn;
+extern u8 PhysBits;
+extern int LongMode;
 
 static inline int runningOnQEMU(void) {
     return CONFIG_QEMU || (
diff --git a/src/fw/paravirt.c b/src/fw/paravirt.c
index c880cb10a1bc..a84968661aee 100644
--- a/src/fw/paravirt.c
+++ b/src/fw/paravirt.c
@@ -32,6 +32,10 @@
 u32 RamSize;
 // Amount of continuous ram >4Gig
 u64 RamSizeOver4G;
+// physical address space bits
+u8 PhysBits;
+// 64bit processor
+int LongMode;
 // Type of emulator platform.
 int PlatformRunningOn VARFSEG;
 // cfg enabled
@@ -129,6 +133,58 @@ static void kvmclock_init(void)
     tsctimer_setfreq(MHz * 1000, "kvmclock");
 }
 
+static void physbits(int qemu_quirk)
+{
+    unsigned int max, eax, ebx, ecx, edx;
+    unsigned int physbits;
+    char signature[13];
+    int pae = 0, valid = 0;
+
+    cpuid(0, &eax, &ebx, &ecx, &edx);
+    memcpy(signature + 0, &ebx, 4);
+    memcpy(signature + 4, &edx, 4);
+    memcpy(signature + 8, &ecx, 4);
+    signature[12] = 0;
+    if (eax >= 1) {
+        cpuid(1, &eax, &ebx, &ecx, &edx);
+        pae = (edx & (1 << 6));
+    }
+
+    cpuid(0x80000000, &eax, &ebx, &ecx, &edx);
+    max = eax;
+
+    if (max >= 0x80000001) {
+        cpuid(0x80000001, &eax, &ebx, &ecx, &edx);
+        LongMode = (edx & (1 << 29));
+    }
+
+    if (pae && LongMode && max >= 0x80000008) {
+        cpuid(0x80000008, &eax, &ebx, &ecx, &edx);
+        physbits = (u8)eax;
+        if (!qemu_quirk) {
+            valid = 1;
+        } else if (physbits >= 41) {
+            valid = 1;
+        } else if (strcmp(signature, "GenuineIntel") == 0) {
+            if ((physbits == 36) || (physbits == 39))
+                valid = 1;
+        } else if (strcmp(signature, "AuthenticAMD") == 0) {
+            if (physbits == 40)
+                valid = 1;
+        }
+    } else {
+        physbits = pae ? 36 : 32;
+        valid = 1;
+    }
+
+    dprintf(1, "%s: signature=\"%s\", pae=%s, lm=%s, phys-bits=%d, valid=%s\n",
+            __func__, signature, pae ? "yes" : "no", LongMode ? "yes" : "no",
+            physbits, valid ? "yes" : "no");
+
+    if (valid)
+        PhysBits = physbits;
+}
+
 static void qemu_detect(void)
 {
     if (!CONFIG_QEMU_HARDWARE)
@@ -161,6 +217,7 @@ static void qemu_detect(void)
         dprintf(1, "Running on QEMU (unknown nb: %04x:%04x)\n", v, d);
         break;
     }
+    physbits(1);
 }
 
 static int qemu_early_e820(void);
-- 
2.40.1

On Wed, May 03, 2023 at 11:20:54AM +0200, Gerd Hoffmann wrote:
> Check for pae and long mode using cpuid.  If present also read the
> physical address bits.  Apply some qemu sanity checks (see below).
> Record results in PhysBits and LongMode variables.  In case we are not
> sure what the address space size is leave the PhysBits variable unset.
> 
> On qemu we have the problem that for historical reasons x86_64
> processors advertise 40 physical address space bits by default, even in
> case the host supports less than that so actually using the whole
> address space will not work.
> 
> Because of that the code applies some extra sanity checks in case we
> find 40 (or less) physical address space bits advertised.  Only
> known-good values (which is 40 for amd processors and 36+39 for intel
> processors) will be accepted as valid.
> 
> Recommendation is to use 'qemu -cpu ${name},host-phys-bits=on' to
> advertise valid physical address space bits to the guest.  Some distro
> builds enable this by default, and most likely the qemu default will
> change in near future too.
> 
> Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
> ---
>  src/fw/paravirt.h |  2 ++
>  src/fw/paravirt.c | 57 +++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 59 insertions(+)
> 
> diff --git a/src/fw/paravirt.h b/src/fw/paravirt.h
> index 4e2e993ba9d3..7ca3fdcc06d4 100644
> --- a/src/fw/paravirt.h
> +++ b/src/fw/paravirt.h
> @@ -31,6 +31,8 @@ typedef struct QemuCfgDmaAccess {
>  extern u32 RamSize;
>  extern u64 RamSizeOver4G;
>  extern int PlatformRunningOn;
> +extern u8 PhysBits;
> +extern int LongMode;
>  
>  static inline int runningOnQEMU(void) {
>      return CONFIG_QEMU || (
> diff --git a/src/fw/paravirt.c b/src/fw/paravirt.c
> index c880cb10a1bc..a84968661aee 100644
> --- a/src/fw/paravirt.c
> +++ b/src/fw/paravirt.c
> @@ -32,6 +32,10 @@
>  u32 RamSize;
>  // Amount of continuous ram >4Gig
>  u64 RamSizeOver4G;
> +// physical address space bits
> +u8 PhysBits;
> +// 64bit processor
> +int LongMode;

Odd that LongMode is an "int" while PhysBits is a u8..  Would probably
be simpler to do: LongMode = !!(edx & (1 << 29))  - and then treat it as
a flag variable.

Since these variables are exported, it might also be worthwhile to
give them more descriptive names - for example CPUPhysBits and
CPULongMode.

Separately, series looks fine to me.
-Kevin


>  // Type of emulator platform.
>  int PlatformRunningOn VARFSEG;
>  // cfg enabled
> @@ -129,6 +133,58 @@ static void kvmclock_init(void)
>      tsctimer_setfreq(MHz * 1000, "kvmclock");
>  }
>  
> +static void physbits(int qemu_quirk)
> +{
> +    unsigned int max, eax, ebx, ecx, edx;
> +    unsigned int physbits;
> +    char signature[13];
> +    int pae = 0, valid = 0;
> +
> +    cpuid(0, &eax, &ebx, &ecx, &edx);
> +    memcpy(signature + 0, &ebx, 4);
> +    memcpy(signature + 4, &edx, 4);
> +    memcpy(signature + 8, &ecx, 4);
> +    signature[12] = 0;
> +    if (eax >= 1) {
> +        cpuid(1, &eax, &ebx, &ecx, &edx);
> +        pae = (edx & (1 << 6));
> +    }
> +
> +    cpuid(0x80000000, &eax, &ebx, &ecx, &edx);
> +    max = eax;
> +
> +    if (max >= 0x80000001) {
> +        cpuid(0x80000001, &eax, &ebx, &ecx, &edx);
> +        LongMode = (edx & (1 << 29));
> +    }
> +
> +    if (pae && LongMode && max >= 0x80000008) {
> +        cpuid(0x80000008, &eax, &ebx, &ecx, &edx);
> +        physbits = (u8)eax;
> +        if (!qemu_quirk) {
> +            valid = 1;
> +        } else if (physbits >= 41) {
> +            valid = 1;
> +        } else if (strcmp(signature, "GenuineIntel") == 0) {
> +            if ((physbits == 36) || (physbits == 39))
> +                valid = 1;
> +        } else if (strcmp(signature, "AuthenticAMD") == 0) {
> +            if (physbits == 40)
> +                valid = 1;
> +        }
> +    } else {
> +        physbits = pae ? 36 : 32;
> +        valid = 1;
> +    }
> +
> +    dprintf(1, "%s: signature=\"%s\", pae=%s, lm=%s, phys-bits=%d, valid=%s\n",
> +            __func__, signature, pae ? "yes" : "no", LongMode ? "yes" : "no",
> +            physbits, valid ? "yes" : "no");
> +
> +    if (valid)
> +        PhysBits = physbits;
> +}
> +
>  static void qemu_detect(void)
>  {
>      if (!CONFIG_QEMU_HARDWARE)
> @@ -161,6 +217,7 @@ static void qemu_detect(void)
>          dprintf(1, "Running on QEMU (unknown nb: %04x:%04x)\n", v, d);
>          break;
>      }
> +    physbits(1);
>  }
>  
>  static int qemu_early_e820(void);
> -- 
> 2.40.1
> 
> 
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