Re: [Qemu-devel][PATCH] x86/cpu: Use max host physical address if -cpu max option is applied

[Paolo Bonzini <pbonzini@redhat.com>]

On 25/01/21 08:10, Nathan Chancellor wrote:
>>> This patch as commit 5a140b255d ("x86/cpu: Use max host physical address
>>> if -cpu max option is applied") prevents me from using '-cpu host' while
>>> booting an i386_defconfig kernel.
>>>
>>> $ qemu-system-i386 \
>>> -append console=ttyS0 \
>>> -cpu host \
>>> -display none \
>>> -enable-kvm \
>>> -initrd rootfs.cpio \
>>> -kernel bzImage \
>>> -serial mon:stdio
>>> qemu-system-i386: phys-bits should be between 32 and 36  (but is 48)
>>>
>>> Am I expected to pass "-cpu host,host-phys-bits=false" now or did this
>>> do something unexpected?

Yes, it's setting the LM bit for a 32-bit guest.

Does this work for you?

diff --git a/target/i386/cpu.c b/target/i386/cpu.c
index 72a79e6019..70df57337f 100644
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -5081,6 +5081,11 @@ static uint64_t 
x86_cpu_get_supported_feature_word(FeatureWord w,
      } else {
          return ~0;
      }
+#ifdef TARGET_I386
+    if (wi->cpuid.eax = 0x80000001) {
+        r &= ~CPUID_EXT2_LM;
+    }
+#endif
      if (migratable_only) {
          r &= x86_cpu_get_migratable_flags(w);
      }


Paolo

>> Hi, Nathan,
>> Could you try Paolo's latest patch?
>>
>> [PULL 03/31] x86/cpu: Use max host physical address if -cpu max option is applied
> 
> Hi Yang,
> 
> That is the version of the patch I tried, which has been pulled into the
> master branch.
> 
> Cheers,
> Nathan
>