[Xen-devel] [RFC] xsave/xrstor: changes to the struct vcpu_guest_context

["Cui, Dexuan" <dexuan.cui@xxxxxxxxx>]

Hi all,
I'm working to add the xsave/xrstor support for Xen.
The related Intel SDMs are at http://www.intel.com/products/processor/manuals/
And AVX programming reference is at http://software.intel.com/en-us/avx/

After enabling xsave/xrstor in xen, with respect to guest save/restore(S/R) we 
need to save more cpu-related states besides FPU/SSE, e.g., we need to save the 
xsave_bv_header, and a new state like YMM state, and even more new states in 
the future.
To not break guest S/R and keep the compatibility, I think we should make the 
changes to the struct vcpu_guest_context as the attached example patch does:

The changes:
1)  define a new struct vcpu_guest_context_new: its first feild is the legacy 
struct vcpu_guest_context and the other is the new xsave field;
2) add a new flag (a new bit definition) VGCF_xsave_valid to the existing 
'flags' field;

My proposal:
During guest S/R, when we transfer the struct between tools and xen, we *will 
always* use the new expanded struct vcpu_guest_context_new:
        3.1) in tools
                3.1.1) when restore:  first we only read the leading part of 
the struct (namely, the legacy struct vcpu_guest_context); if VGCF_xsave_valid 
is set, we further read the new xsave field;
                3.1.2) when save:  if VGCF_xsave_valid is set, we save the 
whole expanded struct vcpu_guest_context_new; else, we only save the legacy 
part;
        3.2) in xen
                3.2.1) when restore: if VGCF_xsave_valid is set, use the whole 
expaned struct vcpu_guest_context_new; else, only use the legacy part;
                3.2.2) when save: if the host supports xsave and the guest 
enables xsave, xen sets VGCF_xsave_valid and fills the whole expanded struct 
vcpu_guest_context_new;

In this way:
1) guest img saved in old xen, e.g. xen-3.2
        1.1) can be restored in xen-3.2 on xsave-{capable and incapble} host;
        1.2) can be restored in xen-unstable on xsave-{capable and incapble} 
host;

2) guest img saved in xen-unstable
        2.1) if the host doesn't support xsave, the saved img can be restored 
in xen-3.2 or 'older' xen-unstable (namely, before the xsave patches are 
checked in) on xsave-{capable and incapable} hosts;
        2.2) if the host does support xsave
                2.2.1) if guest doesn't use xsave, the saved img can be 
restored in xen-3.2 or older xen-unstable on xsave-{capable and incapable} 
hosts;
                2.2.2) if guest does use xsave, the saved img can NOT be 
restored in xen-3.2 or older xen-unstable, and the saved image can NOT be 
restored in the new xen-unstable (namely, after the xsave patches are checked 
in) on xsave-incapable host.


BTW, for live migration: the rule and handling should be the same.

Could you please comment on this?  Thanks!

-- Dexuan

Attachment: vcpu_guest_context.patch
Description: vcpu_guest_context.patch

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