RE: [Xen-devel] [PATCH 06/16] vmx: nest: handling VMX instruction exits

["Dong, Eddie" <eddie.dong@xxxxxxxxx>]

Keir Fraser wrote:
> On 15/09/2010 10:08, "Dong, Eddie" <eddie.dong@xxxxxxxxx> wrote:
> 
>>> But yes I can see that emulation of L2 guest instructions is needed
>>> in some other cases. Like instructions performing I/O in areas which
>>> L1 thinks it has given L2 direct unmediated access to, but which Xen
>>> is actually filtering or emulating.
>> 
>> That may be an issue in plan when we support virtual VT-d for nested
>> I/O performance. But not now :)
> 
> Actually it is an issue now. This has nothing to do with VT-d (ie.
> IOMMU, irq remapping, etc) but with basic core VMX functionality --
> per I/O port direct execute versus vmexit; per virtual-address page

I see, for the I/O port, right now we are letting L1 handle it though it 
doesn't expect to :(
How about to remove the capability of CPU_BASED_ACTIVATE_IO_BITMAP in L1 VMM 
for now to focus on framework?


> direct access versus #PF vmexit; per physical-frame direct access
> versus nexted-paging vmexit. In any of these cases the L1 may think

Didn't quit catch. The memory direct access is always guarded by L0 shadow or 
nested EPT/NPT. Missing something?

> it has given direct unfettered access to the L2, but L0 (Xen) is
> actually blocking it. In this case any resulting required instruction
> emulations have to be performed by Xen on behalf of the L2, without
> L1's help or knowledge. Consider that even in Xen we give all HVM
> guests direct access to thinks like port 0x80 for perf reasons. Maybe
> a VMM running in L1 would give direct access to more even than that
> -- in such cases Xen must be able to emulate those 'direct' accesses. 

Agree!

> 
> Now this shouldn't be hard to arrange anyhow. When you vmexit to Xen
> from running L2 guest, the saved general-purpose CPU state will be
> the L2's state, and that is what you would want x86_emulate to see.
> But it does require some thought, and it is merely not an extension
> to be dealt with later. It is core VMX stuff and hence core nested
> VMX stuff. 

Yes. For I/O issue, emulating by L0 should be fine.

Thx, Eddie
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