Re: [Xen-devel] [PATCH RFC v13 06/20] pvh: vmx-specific changes

[George Dunlap <george.dunlap@xxxxxxxxxxxxx>]

On 08/10/13 09:45, Jan Beulich wrote:
> > > > On 07.10.13 at 18:12, Tim Deegan <tim@xxxxxxx> wrote:
> > At 17:06 +0100 on 07 Oct (1381165615), George Dunlap wrote:
> > > On 07/10/13 16:55, Roger Pau Monnà wrote:
> > > > On 23/09/13 18:49, George Dunlap wrote:
> > > > > @@ -1028,12 +1129,28 @@ static int construct_vmcs(struct vcpu *v)
> > > > >                 | (1U << TRAP_no_device);
> > > > >       vmx_update_exception_bitmap(v);
> > > > >   
> > > > > +    /* In HVM domains, this happens on the realmode->paging
> > > > > +     * transition.  Since PVH never goes through this transition, we
> > > > > +     * need to do it at start-of-day. */
> > > > > +    if ( is_pvh_domain(d) )
> > > > > +        vmx_update_debug_state(v);
> > > > > +
> > > > >       v->arch.hvm_vcpu.guest_cr[0] = X86_CR0_PE | X86_CR0_ET;
> > > > > +
> > > > > +    /* PVH domains always start in paging mode */
> > > > > +    if ( is_pvh_domain(d) )
> > > > > +        v->arch.hvm_vcpu.guest_cr[0] |= X86_CR0_PG | X86_CR0_NE |
> > > > > X86_CR0_WP;
> > > > > +
> > > > >       hvm_update_guest_cr(v, 0);
> > > > >   
> > > > > -    v->arch.hvm_vcpu.guest_cr[4] = 0;
> > > > > +    v->arch.hvm_vcpu.guest_cr[4] = is_pvh_domain(d) ?
> > > > > +        real_cr4_to_pv_guest_cr4(mmu_cr4_features)
> > > > Here we need to mask the bits in CR4 that the guest isn't allowed to
> > > > set. Right now Xen is setting the VMXE bit by default, which the guest
> > > > is not able to modify, so if the guests tries to update CR4 based on the
> > > > previous value Xen is going to complain:
> > > >
> > > > +        real_cr4_to_pv_guest_cr4(mmu_cr4_features) &
> > > > +        ~HVM_CR4_GUEST_RESERVED_BITS(v)
> > > Thanks for testing that -- I'll include it in the next spin-up.
> > <harping on> This is the sort of thing that would be easier if PVH guests
> > were just HVM ones with extra hypercalls. <harping off>
> And indeed I'm rather puzzled by the simplistic re-use of a PV
> construct: There are memory management related bits in CR4,
> and with memory management not being PV for PVH guests,
> this seems conceptually wrong.
What would be a better approach, do you think?

 -George

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