Re: [PATCH v13 00/85] KVM: Stop grabbing references to PFNMAP'd pages

Dmitry Osipenko
2024-10-24T03:39:52Z

On 10/10/24 21:23, Sean Christopherson wrote:
> TL;DR: Eliminate KVM's long-standing (and heinous) behavior of essentially
> guessing which pfns are refcounted pages (see kvm_pfn_to_refcounted_page()).
>
> Getting there requires "fixing" arch code that isn't obviously broken.
> Specifically, to get rid of kvm_pfn_to_refcounted_page(), KVM needs to
> stop marking pages/folios dirty/accessed based solely on the pfn that's
> stored in KVM's stage-2 page tables.
>
> Instead of tracking which SPTEs correspond to refcounted pages, simply
> remove all of the code that operates on "struct page" based ona the pfn
> in stage-2 PTEs. This is the back ~40-50% of the series.
>
> For x86 in particular, which sets accessed/dirty status when that info
> would be "lost", e.g. when SPTEs are zapped or KVM clears the dirty flag
> in a SPTE, foregoing the updates provides very measurable performance
> improvements for related operations. E.g. when clearing dirty bits as
> part of dirty logging, and zapping SPTEs to reconstitue huge pages when
> disabling dirty logging.
>
> The front ~40% of the series is cleanups and prep work, and most of it is
> x86 focused (purely because x86 added the most special cases, *sigh*).
> E.g. several of the inputs to hva_to_pfn() (and it's myriad wrappers),
> can be removed by cleaning up and deduplicating x86 code.
>
> v13:
> - Rebased onto v6.12-rc2
> - Collect reviews. [Alex and others]
> - Fix a transient bug in arm64 and RISC-V where KVM would leak a page
> refcount. [Oliver]
> - Fix a dangling comment. [Alex]
> - Drop kvm_lookup_pfn(), as the x86 that "needed" it was stupid and is (was?)
> eliminated in v6.12.
> - Drop check_user_page_hwpoison(). [Paolo]
> - Drop the arm64 MTE fixes that went into 6.12.
> - Slightly redo the guest_memfd interaction to account for 6.12 changes.

Thanks a lot for working on this patchset! I tested it with native
amdgpu/intel contexts and venus/virgl with dGPU and iGPU, no problems
spotted. Please merge sooner, this will unblock lots of new virtio-gpu
features.

Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>

--
Best regards,
Dmitry