RE: [Xen-devel] Question about PV kernel ring

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Ian Campbell
> Sent: Tuesday, May 31, 2011 5:10 PM
> 
> On Mon, 2011-05-30 at 18:23 +0100, Liu, Jinsong wrote:
> > Keir,
> >
> > I have a question about pv kernel ring arrangement:
> >
> > arch/x86/x86_32/traps.c
> > void hypercall_page_initialise(struct domain *d, void *hypercall_page)
> > {
> >     memset(hypercall_page, 0xCC, PAGE_SIZE);
> >     if ( is_hvm_domain(d) )
> >         hvm_hypercall_page_initialise(d, hypercall_page);
> >     else if ( supervisor_mode_kernel )
> >         hypercall_page_initialise_ring0_kernel(hypercall_page);
> >     else
> >         hypercall_page_initialise_ring1_kernel(hypercall_page);
> > }
> >
> > arch/x86/x86_64/traps.c
> > void hypercall_page_initialise(struct domain *d, void *hypercall_page)
> > {
> >     memset(hypercall_page, 0xCC, PAGE_SIZE);
> >     if ( is_hvm_domain(d) )
> >         hvm_hypercall_page_initialise(d, hypercall_page);
> >     else if ( !is_pv_32bit_domain(d) )
> >         hypercall_page_initialise_ring3_kernel(hypercall_page);
> >     else
> >         hypercall_page_initialise_ring1_kernel(hypercall_page);
> > }
> >
> > My question:
> > 1. for x86_32 hypervisor, what's the purpose and advantage/disadvantage of
> supervisor_mode_kernel pv which runs at ring0?
> 
> supervisor_mode_kernel was a proof of concept project about 5 years ago
> to run a Xen PV kernel on a thin "hypervisor" shim. It provides no
> actual virtualisation features (i.e. multiple domains) and there is no
> protection between the kernel and the hypervisor shim. It was mostly a
> stunt to see what the minimum amount of scaffolding to support a PV
> kernel might be, it was kind of the skanky opposite approach to pvops I
> guess. IOW the disadvantages far outweigh the advantages.

Time to remove it then, since no one actually uses it today and nobody knows 
whether it still works given its limited value?

> 
> > 2. for x86_64 hypervisor, why no supervisor_mode_kernel pv? seems pv can
> also do so ...
> 
> I don't recall if supervisor_mode_kernel ever worked for 64 (and has
> since bit-rotted and be removed) or if it was never written for 64 bit
> in the first place. Either way it doesn't exist now and there would be
> very little point in writing it.
> 

It's a reasonable result. Once the basic verification is done on 32bit Xen, 
there's
no point to do same experiment on 64bit when people all understand the 
architecture well. :-)

Thanks
Kevin

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel