On Mon, Jul 22, 2013 at 12:50:23PM -0500, Anthony Liguori wrote:
> Paolo Bonzini <pbonz...@redhat.com> 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 <owass...@redhat.com>
> >> ---
> >> 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.
