Re: [Xen-devel] [PATCH 4/5] libxc/pvh: set default MTRR type to write-back

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, May 14, 2018 at 05:42:53PM +0100, Wei Liu wrote:
> On Mon, May 14, 2018 at 05:02:47PM +0100, Roger Pau Monné wrote:
> > On Mon, May 14, 2018 at 05:00:51PM +0100, Wei Liu wrote:
> > > On Thu, May 10, 2018 at 06:15:04PM +0100, Roger Pau Monne wrote:
> > > > @@ -1014,6 +1034,30 @@ static int vcpu_hvm(struct xc_dom_image *dom)
> > > >      if ( dom->start_info_seg.pfn )
> > > >          bsp_ctx.cpu.rbx = dom->start_info_seg.pfn << PAGE_SHIFT;
> > > >  
> > > > +    /* Set the MTRR. */
> > > > +    bsp_ctx.mtrr_d.typecode = HVM_SAVE_CODE(MTRR);
> > > > +    bsp_ctx.mtrr_d.instance = 0;
> > > > +    bsp_ctx.mtrr_d.length = HVM_SAVE_LENGTH(MTRR);
> > > > +
> > > > +    mtrr_record = hvm_get_save_record(full_ctx, HVM_SAVE_CODE(MTRR), 
> > > > 0);
> > > > +    if ( !mtrr_record )
> > > > +    {
> > > > +        xc_dom_panic(dom->xch, XC_INTERNAL_ERROR,
> > > > +                     "%s: unable to get MTRR save record", __func__);
> > > > +        goto out;
> > > > +    }
> > > 
> > > Will this break cross version migration when the older hypervisor
> > > doesn't have such record?
> > 
> > This migration record is already present, it's not introduced in this
> > patch series. I'm simply making use of it in order to set a valid
> > initial MTRR state.
> > 
> 
> Then I'm even more confused: does this mean the record is not properly
> honoured in the hypervisor? I.e. this is a hypervisor bug?

I'm kind of lost. This code I'm adding is not used for migration at
all. This is used in order to set the initial vCPU state for PVH
guests (which is done using xc_domain_hvm_setcontext). In order to
provide an initial MTRR state we need to create a MTRR record and
provide it to the hypervisor. I'm not introducing the MTRR record in
this patch, I'm just making use of it in order to set a sane initial
MTRR state for PVH guests.

libxc already does the same in order to set the initial CPU register
state for PVH/HVM guests.

Thanks, Roger.

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