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SubjectRe: [RFC PATCH 00/13] XOM for KVM guest userspace
On Thu, Oct 3, 2019 at 2:38 PM Rick Edgecombe
<rick.p.edgecombe@intel.com> 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.

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