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

["Liu, Jinsong" <jinsong.liu@xxxxxxxxx>]

Ian Campbell wrote:
> 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.

I see, thanks Ian!
We just meet a little issue need confirm pv kernel ring ... :)

Jinsong
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