Re: [PATCH 09/13] KVM: x86: save/load state on SMM switch

["Radim Krčmář" <rkrcmar@redhat.com>]

2015-05-05 11:37+0200, Paolo Bonzini:
> On 04/05/2015 21:59, Radim Krčmář wrote:
> > > The big ugly one.  This patch adds support for switching in and out of
> > > system management mode, respectively upon receiving KVM_REQ_SMI and upon
> > > executing a RSM instruction.  Both 32- and 64-bit formats are supported
> > > for the SMM state save area.
> > > 
> > > Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> > > ---
> > > diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> > > +static void rsm_set_desc_flags(struct desc_struct *desc, u16 flags)
> > > +{
> > > +	desc->g    = (flags >> 15) & 1;
> > > +	desc->d    = (flags >> 14) & 1;
> > > +	desc->l    = (flags >> 13) & 1;
> > > +	desc->avl  = (flags >> 12) & 1;
> > > +	desc->p    = (flags >> 7) & 1;
> > > +	desc->dpl  = (flags >> 5) & 3;
> > > +	desc->s    = (flags >> 4) & 1;
> > > +	desc->type = flags & 15;
> >
> > I can't find a description of this ... can you point me to a place where
> > the gap between 'p' and 'avl' is documented?
> > (Not that it matters unless the guest reads it, but it's a bit weird.)
> 
> It turns out that access rights are stored in the same format as the VMX
> access rights.

Thanks, so it really has a "reserved" space in the middle, to save some
processing because the format is horrible (backward compatible).

> access rights.  However, they are shifted by 8, which my code above
> doesn't do (bug).

I think you are shifting it right, though ... they wouldn't fit into a
word if shifted left.

(I'd just shorten it after finding the right name for mask
   u32 mask = 0x00f0ff00
   desc->b = desc->b & ~mask | flags << 8 & mask

 and maybe define 'attributes' field in desc that is already shifted.)

> The documentation is, of course, QEMU and Bochs :) but you can also find
> it in http://www.rcollins.org/ftp/source/include/struc.inc.  It is not
> exactly for SMM, but it is more or less the same.

(It's a register hidden from software, so I have some trust issues :])