Re: [Qemu-devel] [PATCH 1/2] Fix real mode guest migration

[Paolo Bonzini]

Il 22/07/2013 08:49, Orit Wasserman ha scritto:
> Older KVM versions save CS dpl value to an invalid value for real mode guests
> (0x3). This patch detect this situation when loading CPU state and set all the
> segments dpl to zero.
> This will allow migration from older KVM on host without unrestricted guest
> to hosts with restricted guest support.
> For example migration from a Penryn host (with kernel 2.6.32) to
> a Westmere host.
> 
> Signed-off-by: Orit Wasserman <address@hidden>
> ---
>  target-i386/machine.c | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/target-i386/machine.c b/target-i386/machine.c
> index 3659db9..7e95829 100644
> --- a/target-i386/machine.c
> +++ b/target-i386/machine.c
> @@ -260,6 +260,24 @@ static int cpu_post_load(void *opaque, int version_id)
>      CPUX86State *env = &cpu->env;
>      int i;
>  
> +    /*
> +      Real mode guest segments register DPL should be zero.
> +      Older KVM version were setting it worngly.
> +      Fixing it will allow live migration from such host that don't have
> +      restricted guest support to an host with unrestricted guest support
> +      (otherwise the migration will fail with invalid guest state
> +      error).
> +    */

Coding standard asks for *s on every line.

As discussed offlist, I would prefer to have this in the kernel since
that's where the bug is.  Gleb disagrees.

We need to find a third person who mediates...  Anthony, Eduardo, what
do you think?

Paolo

> +    if (!(env->cr[0] & CR0_PE_MASK) &&
> +         (env->segs[R_CS].flags >> DESC_DPL_SHIFT & 3) != 0) {
> +        env->segs[R_CS].flags &= ~(env->segs[R_CS].flags & DESC_DPL_MASK);
> +        env->segs[R_DS].flags &= ~(env->segs[R_DS].flags & DESC_DPL_MASK);
> +        env->segs[R_ES].flags &= ~(env->segs[R_ES].flags & DESC_DPL_MASK);
> +        env->segs[R_FS].flags &= ~(env->segs[R_FS].flags & DESC_DPL_MASK);
> +        env->segs[R_GS].flags &= ~(env->segs[R_GS].flags & DESC_DPL_MASK);
> +        env->segs[R_SS].flags &= ~(env->segs[R_SS].flags & DESC_DPL_MASK);
> +    }
> +
>      /* XXX: restore FPU round state */
>      env->fpstt = (env->fpus_vmstate >> 11) & 7;
>      env->fpus = env->fpus_vmstate & ~0x3800;
>