On Tue, Jan 31, 2023 at 01:18:15AM +0000, Sean Christopherson wrote: > On Mon, Jan 30, 2023, Oliver Upton wrote: > > I think that Marc's suggestion of having userspace configure this is > > sound. After all, userspace _should_ know the granularity of the backing > > source it chose for guest memory. > > > > We could also interpret a cache size of 0 to signal that userspace wants > > to disable eager page split for a VM altogether. It is entirely possible that > > the user will want a differing QoS between slice-of-hardware and > > overcommitted VMs. > > Maybe. It's also entirely possible that QoS is never factored in, e.g. if QoS > guarantees for all VMs on a system are better met by enabling eager splitting > across the board. > > There are other reasons to use module/kernel params beyond what Marc listed, e.g. > to let the user opt out even when something is on by default. x86's TDP MMU has > benefited greatly from downstream users being able to do A/B performance testing > this way. I suspect x86's eager_page_split knob was added largely for this > reason, e.g. to easily see how a specific workload is affected by eager splitting. > That seems like a reasonable fit on the ARM side as well. There's a rather important distinction here in that we'd allow userspace to select the page split cache size, which should be correctly sized for the backing memory source. Considering the break-before-make rules of the architecture, the only way eager split is performant on arm64 is by replacing a block entry with a fully populated table hierarchy in one operation. AFAICT, you don't have this problem on x86, as the architecture generally permits a direct valid->valid transformation without an intermediate invalidation. Well, ignoring iTLB multihit :) So, the largest transformation we need to do right now is on a PUD w/ PAGE_SIZE=4K, leading to 513 pages as proposed in the series. Exposing that configuration option in a module parameter is presumptive that all VMs on a host use the exact same memory configuration, which doesn't feel right to me. -- Thanks, Oliver
