Re: [PATCH v3 8/9] KVM: x86/mmu: don't zap secondary MMU on permission upgrade

[sashiko-bot@kernel.org]

> When an mmu_notifier invalidation only upgrades permissions
> (MMU_NOTIFIER_RANGE_PERM_UPGRADE), keep the existing SPTEs instead of
> zapping them. The PFN is unchanged and no access is revoked, so an
> existing SPTE is no more permissive than the new host PTE, and the new
> permissions are picked up on the next fault.
> 
> The motivating case is a userfaultfd-wp dirty tracker such as QEMU's
> background snapshot, which unprotects every guest page as it saves it.
> Zapping on each unprotect makes the guest refault its whole working
> set. On a 64GB guest under a background snapshot, a read-mostly workload
> drops secondary-MMU refaults from ~1M/s to ~2.5K/s and cross-vCPU TLB
> shootdowns from ~1400/s to near zero.
> 
> Note: this also skips the APIC-access page reload, which is not needed
> as the page's PFN is unchanged.
> 
> Signed-off-by: Carlos López <clopez@suse.de>

Sashiko has reviewed this patch and found no issues.

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Sashiko AI review · https://sashiko.dev/#/patchset/20261112142039.907090-2-clopez@suse.de?part=8