On 19.05.23 12:10, Marek Marczykowski-Górecki wrote:
On Fri, May 19, 2023 at 06:04:05AM +0200, Christoph Hellwig wrote:On Thu, May 18, 2023 at 08:18:39PM +0200, Marek Marczykowski-Górecki wrote:On Thu, May 18, 2023 at 03:42:51PM +0200, Christoph Hellwig wrote:Remove the dangerous late initialization of xen-swiotlb in pci_xen_swiotlb_init_late and instead just always initialize xen-swiotlb in the boot code if CONFIG_XEN_PCIDEV_FRONTEND is enabled.Signed-off-by: Christoph Hellwig <h...@lst.de>Doesn't it mean all the PV guests will basically waste 64MB of RAM by default each if they don't really have PCI devices?If CONFIG_XEN_PCIDEV_FRONTEND is enabled, and the kernel's isn't booted with swiotlb=noforce, yes.That's "a bit" unfortunate, since that might be significant part of the VM memory, or if you have a lot of VMs, a significant part of the host memory - it quickly adds up. While I would say PCI passthrough is not very common for PV guests, can the decision about xen-swiotlb be delayed until you can enumerate xenstore to check if there are any PCI devices connected (and not allocate xen-swiotlb by default if there are none)? This would still not cover the hotplug case (in which case, you'd need to force it with a cmdline), but at least you wouldn't loose much memory just because one of your VMs may use PCI passthrough (so, you have it enabled in your kernel). Please remember that guest kernel is not always under full control of the host admin, so making guests loose 64MB of RAM always, in default setup isn't good for customers of such VMs...
In normal cases PCI passthrough in PV guests requires to start the guest with e820_host=1. So it should be rather easy to limit allocating the 64MB in PV guests to the cases where the memory map has non-RAM regions especially in the first 1MB of the memory. This will cover even hotplug cases. The only case not covered would be a guest started with e820_host=1 even if no PCI passthrough was planned. But this should be rather rare (at least I hope so). Juergen
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