Re: [PATCH v3 0/2] x86: Add Support for Paging-Write Feature

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

On 1/9/25 4:25 PM, Tamas K Lengyel wrote:
> On Thu, Jan 9, 2025 at 9:30 AM Oleksii Kurochko
> <oleksii.kurochko@xxxxxxxxx> wrote:
> > On 1/2/25 6:13 PM, Petr Beneš wrote:
> >
> > From: Petr Beneš <w1benny@xxxxxxxxx>
> >
> > Changes since v2:
> > - Reset entry->pw in all cases in p2m_set_entry, except for p2m_access_r_pw
> >
> > Changes since v1:
> > - Added signed-off-by tags
> >
> > This patch introduces a new XENMEM_access_r_pw permission. Functionally, it is 
> > similar to XENMEM_access_r, but for processors with 
> > TERTIARY_EXEC_EPT_PAGING_WRITE support (Intel 12th Gen/Alder Lake and later), 
> > it also permits the CPU to write to the page during guest page-table walks 
> > (e.g., updating A/D bits) without triggering an EPT violation.
> >
> > This behavior works by both enabling the EPT paging-write feature and setting 
> > the EPT paging-write flag in the EPT leaf entry.
> >
> > This feature provides a significant performance boost for introspection tools that monitor guest 
> > page-table updates. Previously, every page-table modification by the guest—including routine 
> > updates like setting A/D bits—triggered an EPT violation, adding unnecessary overhead. The new 
> > XENMEM_access_r_pw permission allows these "uninteresting" updates to occur without EPT 
> > violations, improving efficiency.
> >
> > Considering that this feature provides a significant performance boost for 
> > introspection tools probably we could consider to take it to current release.
> >
> > I see that the patch series was acked-by "Acked-by: Tamas K Lengyel 
> > <tamas@xxxxxxxxxxxxx>" but based on the change log it is not clear when exactly
> >
> > before Feature freeze date or not. ( and I don't see any reply from Tamas ).
> I've acked the patch Thu, Dec 19, 2024.
In this case, I believe we should consider including it in the current 
release, as it was technically ready for merging before the Feature 
Freeze deadline.
I would like to kindly ask the x86 maintainers for clarification on why 
this patch series was not merged.
~ Oleksii