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

[Christoph Egger <Christoph.Egger@xxxxxxx>]

On Monday 20 September 2010 10:08:02 Keir Fraser wrote:
> On 20/09/2010 04:13, "Dong, Eddie" <eddie.dong@xxxxxxxxx> wrote:
> >>>> 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?
> >>
> >> Well. It'd be better if just worked really, wouldn't it? :-) How hard
> >> can it be?
> >
> > You are right. It is easy to do, but we have dillemma to either
> > write-protect guest I/O bitmap page, or have to create the shadow I/O
> > bitmap at each vmresume of L2 guest.
>
> You need that anyway don't you, regardless of whether you are accurately
> deciding whether to inject-to-L1 or emulate-L2 on vmexit to L0? Whether you
> inject or emulate, ports that L1 has disallowed for L2 must be properly
> represented in the shadow I/O bitmap page.

You need to do additional range-checking to determine if the guest actually
touched the IO bitmap page in case Xen uses a super page.

>
> > Currently we are injecting to L1 guest, but may be not correct in theory.
> > For now, VMX can trap L2 guest I/O and emulate them in L0, we can revisit
> > some time later to see if we need write-protection of guest I/O bitmap
> > page :)
>
> Are you suggesting to always emulate instead of always inject-to-L1? That's
> still not accurate virtualisation of this VMX feature.
>
> Hmm... Are you currently setting up to always vmexit on I/O port accesses
> by L2? Even if you are, that doesn't stop you looking at the virtual I/O
> bitmap from in your L0 vmexit handler, and doing the right thing (emulate
> versus inject-to-L1).
>
>  -- Keir
>
> > But, yes, L0 VMM needs to emulate L2 instruction here :)



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