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

[Nathan Chancellor]

On Mon, Jan 25, 2021 at 11:42:43AM +0100, Paolo Bonzini wrote:
> 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?

Yes, it does!

Tested-by: Nathan Chancellor <natechancellor@gmail.com>

> 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) {

This should be a '==':

../target/i386/cpu.c: In function ‘x86_cpu_get_supported_feature_word’:
../target/i386/cpu.c:5085:9: error: suggest parentheses around
assignment used as truth value [-Werror=parentheses]
 5085 |     if (wi->cpuid.eax = 0x80000001) {
      |         ^~
cc1: all warnings being treated as errors

> +        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
> > 
V>