On Fri, Oct 4, 2019 at 1:10 PM Edgecombe, Rick P <rick.p.edgecombe@xxxxxxxxx> wrote: > > On Fri, 2019-10-04 at 07:56 -0700, Andy Lutomirski wrote: > > On Thu, Oct 3, 2019 at 2:38 PM Rick Edgecombe > > <rick.p.edgecombe@xxxxxxxxx> wrote: > > > > > > This patchset enables the ability for KVM guests to create execute-only (XO) > > > memory by utilizing EPT based XO permissions. XO memory is currently > > > supported > > > on Intel hardware natively for CPU's with PKU, but this enables it on older > > > platforms, and can support XO for kernel memory as well. > > > > The patchset seems to sometimes call this feature "XO" and sometimes > > call it "NR". To me, XO implies no-read and no-write, whereas NR > > implies just no-read. Can you please clarify *exactly* what the new > > bit does and be consistent? > > > > I suggest that you make it NR, which allows for PROT_EXEC and > > PROT_EXEC|PROT_WRITE and plain PROT_WRITE. WX is of dubious value, > > but I can imagine plain W being genuinely useful for logging and for > > JITs that could maintain a W and a separate X mapping of some code. > > In other words, with an NR bit, all 8 logical access modes are > > possible. Also, keeping the paging bits more orthogonal seems nice -- > > we already have a bit that controls write access. > > Sorry, yes the behavior of this bit needs to be documented a lot better. I will > definitely do this for the next version. > > To clarify, since the EPT permissions in the XO/NR range are executable, and not > readable or writeable the new bit really means XO, but only when NX is 0 since > the guest page tables are being checked as well. When NR=1, W=1, and NX=0, the > memory is still XO. > > NR was picked over XO because as you say. The idea is that it can be defined > that in the case of KVM XO, NR and writable is not a valid combination, like > writeable but not readable is defined as not valid for the EPT. > Ugh, I see, this is an "EPT Misconfiguration". Oh, well. I guess just keep things as they are and document things better, please. Don't try to emulate. I don't suppose Intel could be convinced to get rid of that in a future CPU and allow write-only memory? BTW, is your patch checking for support in IA32_VMX_EPT_VPID_CAP? I didn't notice it, but I didn't look that hard.
