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

[Paolo Bonzini]

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