[Xen-ia64-devel] [Discussion]: dynamic shared_info_va

[Tristan Gingold <Tristan.Gingold@xxxxxxxx>]

Hi,

currently, shared_info_va (or SHAREDINFO_ADDR) is a constant defined in Xen 
and used (through asm-xsi-offsets.h) by Linux.
I am working on a patch to remove this restriction: Linux will set where to 
place shared_info (and shared_archinfo).

Pros:
* Enable the use of shared_info in VTi.
* Possible optimization in Linux (be addressable through addl instead of movl)
* Allow to relocate Xen without recompiling Linux

Problems:
* Absolute constants must be removed (boring but easy).
* In kernel: do a hypercall to set the address, no other changes expected.

* In Xen: there are few use of shared_archinfo_va: mostly in ivt.S and 
hypercall.S.  There are two possibles implementation:
- shared_info/archinfo is mapped twice (one for Xen and one for Linux).  Not 
really elegant, requires two additionnal TR.
- Xen use Linux mapping.  Therefore an additionnal read is required.  The 
slowdown should be negligeable.  The linux mapping may be either in a percpu 
variable, updated at each context switch or within arch_vcpu struct.  The 
later requires another additionnal read, so the former seems better.

* Bootstrap: Xen may use shared_archinfo before Linux set the address.  Two 
solutions: Xen extract the address from Linux binary, or Xen use a default 
mapping before Linux sets the mapping.  I prefer the later, it seems easier 
to implement (note: to avoid problems the default mapping is not readable by 
Linux).

* interrupt_mask_addr is also an issue.  First it is a security issue, because 
it is used by Xen in CPL=0 and may be modified by Linux.  I propose to change 
the name (and the value!): interrupt_mask_offset.  The base is to be defined 
(either shared_info_addr or interrupt_mask_offset itself).

* Mapping: it seems reasonable to require shared_info be mapped in region 7.  
This simplifies region handling by Xen.  Also Xen should prevent Linux to map 
shared_info in Xen space.

* itc/ptc/itr/ptr from Linux are not allowed within shared_info area, because 
this area is tr-mapped!

Comments are welcome.

Tristan.



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