Re: [PATCH v5 4/7] KVM: s390: Improve unmapping of large ranges

[sashiko-bot@kernel.org]

> When unmapping guest ranges, each individual leaf DAT entry is
> invalidated. When unmapping large ranges, this results in a very large
> amount of invalidations, which is time consuming. Moreover, since the
> unmap callback is called by KVM while holding a spinlock, this also
> leads to stall warnings.
> 
> Improve the situation by invalidating DAT entries higher in the tree,
> instead of each leaf individually.
> 
> For small invalidations (up to 1 segment = 1M of guest memory), and
> special cases (like protected VMs) regardless of the size, the old and
> slow invalidation is performed; all leaf entries are invalidated one by
> one.
> 
> For large invalidations (larger than 1M of guest memory), the new
> strategy is used:
>  - Invalidate prefixes on the whole range, extending start and end to
>    1M granularity
>  - Iterate through the page table pointed to by each pmd and unshadow
>    entries as needed
>  - Detach (invalidate) the pmd
>  - Save storage keys as needed
>  - Invalidate each (now detached) pte by simply overwriting it
>  - Reattach the old pmd to point to the updated page table
> 
> Fixes: a2c17f9270cc ("KVM: s390: New gmap code")
> Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>

Sashiko has reviewed this patch and found no issues.

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Sashiko AI review · https://sashiko.dev/#/patchset/20261029130044.125032-1-imbrenda@linux.ibm.com?part=4