Re: [PATCH] virtio_balloon: batch large descriptor chains for page reporting

[sashiko-bot@kernel.org]

> The virtio_balloon driver reports a scatter-gather list of free pages
> to the VMM so it may discard unused pages.
> 
> For an arm64 kernel with 64 KiB pages, the default reporting and
> allocation orders allow entries ranging from 2 MiB to 512 MiB. If the
> report contains a sufficient number of large entries, we will end up
> with a descriptor chain that represents over 4 GiB of memory. This
> violates virtio spec section 2.7.5.2, which requires that the total
> buffer length of a descriptor chain be at most 4 GiB [1]. A VMM that
> faithfully follows the spec may stop processing the descriptor chain
> when it exceeds 4 GiB, and discard less memory than it could. For
> example, this is the case for the virtio-queue in Rust-VMM [2].
> 
> Batch the requests into descriptor chains of at most U32_MAX bytes
> (4 GiB - 1 byte). We use U32_MAX here so we can conveniently represent
> the batch length in a u32.
> 
> [1] https://docs.oasis-open.org/virtio/virtio/v1.4/virtio-v1.4.html
> [2]
> https://github.com/rust-vmm/rust-vmm/blob/9f7250b1555ebe022ea4133fda676e6311040738/vm-virtio/virtio-queue/src/chain.rs#L183-L190
> 
> Fixes: b0c504f15471 ("virtio-balloon: add support for providing free page reports to host")
> Signed-off-by: Ruben Hakobyan <hruben@meta.com>

Sashiko has reviewed this patch and found no issues.

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