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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

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.

> 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.

Ian.



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