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Re: [Xen-devel] [PATCH] x86/VMX: sanitize VM86 TSS handling

At 01:18 -0700 on 15 Feb (1487121525), Jan Beulich wrote:
> >>> On 14.02.17 at 18:33, <tim@xxxxxxx> wrote:
> >> TBD: Do we really want to re-init the TSS every time we are about to
> >>      use it?
> > 
> > No - I think we should init it when the guest writes the param(s) and
> > leave it at that.  Hvmloader marks it as reserved so the guest should
> > know better than to write to it, and we can't protect it against all
> > the possible ways the guest could break itself.
> > 
> > If you do want to re-init it more often, then I think it would still
> > be better to legacy guests' (lack of a) size limit once, when the guest
> > writes the base param.
> The only problem with this being that at the time the base gets
> written we don't know the size yet (nor whether the guest is
> going to write it), and hence we don't know how must space to
> initialize. The lower limit we enforce on the size (if being set) is
> below the 128 byte default for old guests.

Why not make the lower limit 128?  I'd happily exchange simpler
hypervisor code for the theoretical case of a guest that needs to run
realmode code and cares about those few bytes.



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