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

[Gleb Natapov]

On Mon, Jul 22, 2013 at 12:50:23PM -0500, Anthony Liguori wrote:
> Paolo Bonzini <address@hidden> writes:
> 
> > 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?
> 
> I can rationalize the post_load part.  We may get garbage state via live
> migration, we fix it up.
> 
> The pre_save part troubles me though. Is the kernel actively returning
> invalid space to userspace?  If so, this needs to be fixed.  That's
> clearly a bug.
> 
It is fixed in recent kernels.

> Or is this expected to work around an old kernel?  If so, is the kernel
> advertising that it now handles this state correctly via a capability?
> If not, it probably should.
> 
It is to late to do it now, but the if() here identifies the incorrect
state very accurately.

> Regards,
> 
> Anthony Liguori
> 
> >
> > 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;
> >> 
> 

--
                        Gleb.