[release-1.3] mem-hotplug: enable only for VMs with memory >= 1Gi #12328
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The 1GiB mark is chosen as a tradeoff, it is enough memory for the guest kernel to allocate its internal data structures and to allocate the SWIOTLB. It also means we can memory map all PCI devices as they're memory mapped in the first 1Gi (PCI hole). If there are 32bit devices that need to do DMA, the kernel will allocate memory for the SWIOTLB, which is a bounce buffer to address memory after the 4Gi Mark, the SWIOTLB in Linux at the moment of this commit is 64MB and this causes an issue for the kernel with VMs with very small amounts of memory, if not enough memory is available this can lead to the guest kernel panicking while trying to allocate memory. This SWIOTLB issue is present when setting `maxMemory` (which is set in all VMs when `LiveUpdate` is used as the `vmRolloutStrategy`) in the XML domain of the VM because the kernel will have to allow DMA to happen also for virtio-mem hotplugged memory, which is always mapped after the 4Gi mark and thus always needs SWIOTLB for devices that use 32bit addresses. Signed-off-by: Antonio Cardace <acardace@redhat.com> (cherry picked from commit 6a3004c)
This helps properly testing memory hotplug since Alpine v3.20.1 uses Linux v6. Signed-off-by: Antonio Cardace <acardace@redhat.com> (cherry picked from commit 663937a)
Alpine v3.20 uses Linux v6 which is fully compatible with virtio-mem. Signed-off-by: Antonio Cardace <acardace@redhat.com> (cherry picked from commit 071c465)
This is the minimum amount of memory required by Alpine Linux. Signed-off-by: Antonio Cardace <acardace@redhat.com> (cherry picked from commit 1dec670)
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Manual backport of #12212.
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