Re: [RFC PATCH 00/13] XOM for KVM guest userspace

[Edgecombe, Rick P]

On Fri, 2019-10-04 at 09:22 +0200, Paolo Bonzini wrote:
> On 03/10/19 23:23, Rick Edgecombe wrote:
> > Since software would have previously received a #PF with the RSVD error code
> > set, when the HW encountered any set bits in the region 51 to M, there was
> > some
> > internal discussion on whether this should have a virtual MSR for the OS to
> > turn
> > it on only if the OS knows it isn't relying on this behavior for bit M. The
> > argument against needing an MSR is this blurb from the Intel SDM about
> > reserved
> > bits:
> > "Bits reserved in the paging-structure entries are reserved for future
> > functionality. Software developers should be aware that such bits may be
> > used in
> > the future and that a paging-structure entry that causes a page-fault
> > exception
> > on one processor might not do so in the future."
> > 
> > So in the current patchset there is no MSR write required for the guest to
> > turn
> > on this feature. It will have this behavior whenever qemu is run with
> > "-cpu +xo".
> 
> I think the part of the manual that you quote is out of date.  Whenever
> Intel has "unreserved" bits in the page tables they have done that only
> if specific bits in CR4 or EFER or VMCS execution controls are set; this
> is a good thing, and I'd really like it to be codified in the SDM.
> 
> The only bits for which this does not (and should not) apply are indeed
> bits 51:MAXPHYADDR.  But the SDM makes it clear that bits 51:MAXPHYADDR
> are reserved, hence "unreserving" bits based on just a QEMU command line
> option would be against the specification.  So, please don't do this and
> introduce an MSR that enables the feature.
> 
> Paolo
> 
Hi Paolo,

Thanks for taking a look!

Fair enough, MSR it is.

Rick