Re: [Xen-devel] [PATCH 18/18] PVH xen: introduce vmx_pvh.c

[Mukesh Rathor <mukesh.rathor@xxxxxxxxxx>]

On Fri, 28 Jun 2013 10:44:08 +0100
"Jan Beulich" <JBeulich@xxxxxxxx> wrote:

> >>> On 28.06.13 at 04:28, Mukesh Rathor <mukesh.rathor@xxxxxxxxxx>
> >>> wrote:
> > On Tue, 25 Jun 2013 11:49:57 +0100 "Jan Beulich"
> > <JBeulich@xxxxxxxx> wrote:
> >> > +int vmx_pvh_set_vcpu_info(struct vcpu *v, struct
> >> > vcpu_guest_context *ctxtp) +{
> >> > +    if ( v->vcpu_id == 0 )
> >> > +        return 0;
> >> > +
> >> > +    vmx_vmcs_enter(v);
> >> > +    __vmwrite(GUEST_GDTR_BASE, ctxtp->gdt.pvh.addr);
> >> > +    __vmwrite(GUEST_GDTR_LIMIT, ctxtp->gdt.pvh.limit);
> >> > +    __vmwrite(GUEST_GS_BASE, ctxtp->gs_base_user);
> >> > +
> >> > +    __vmwrite(GUEST_CS_SELECTOR, ctxtp->user_regs.cs);
> >> > +    __vmwrite(GUEST_DS_SELECTOR, ctxtp->user_regs.ds);
> >> > +    __vmwrite(GUEST_ES_SELECTOR, ctxtp->user_regs.es);
> >> > +    __vmwrite(GUEST_SS_SELECTOR, ctxtp->user_regs.ss);
> >> > +    __vmwrite(GUEST_GS_SELECTOR, ctxtp->user_regs.gs);
> >> 
> >> How does this work without also writing the "hidden" register
> >> fields?
> > 
> > This is for bringing up SMP CPUs by the guest, which already has
> > set GDT up so it just needs selectors to be loaded to start the
> > target vcpu.
> 
> That makes no sense to me: Once you VMLAUNCH that vCPU, it'll
> get the hidden register fields loaded from the VMCS, without
> accessing the GDT. If that understanding of mine is wrong, please
> explain how you see things working in more detail.

Re-reading this I realize I misunderstood your question. Sorry. The hidden
fields are set to the default values in vmcs create. That call comes from
the tool stack during domain creation via the do_domctl ---> vcpu_initialise
hcall. That happens during guest creation. Here, the guest is booting and 
bringing up secondary CPU, and wants to set certain fields via 
the do_vcpu_op() -> VCPUOP_initialise hcall.

Hope that clarifies.

Thanks
Mukesh


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