On Mon, Dec 02, 2019 at 02:19:49PM -0800, Sean Christopherson wrote: > On Mon, Dec 02, 2019 at 03:53:15PM -0500, Peter Xu wrote: > > On Mon, Dec 02, 2019 at 11:30:27AM -0800, Sean Christopherson wrote: > > > On Fri, Nov 29, 2019 at 04:34:53PM -0500, Peter Xu wrote: > > > > It's already going to reach 2400 Bytes (which is over half of page > > > > size on 4K page archs), so maybe it's good to have this build-time > > > > check in case it overflows when adding new fields. > > > > > > Please explain why exceeding PAGE_SIZE is a bad thing. I realize it's > > > almost absurdly obvious when looking at the code, but a) the patch itself > > > does not provide that context and b) the changelog should hold up on its > > > own, > > > > Right, I'll enhance the commit message. > > > > > e.g. in a mostly hypothetical case where the allocation of vcpu->run > > > were changed to something else. > > > > And that's why I added BUILD_BUG_ON right beneath that allocation. :) > > My point is that if the allocation were changed to no longer be a > straightforward alloc_page() then someone reading the combined code would > have no idea why the BUILD_BUG_ON() exists. It's a bit ridiculous for > this case because the specific constraints of vcpu->run make it highly > unlikely to use anything else, but that's beside the point. > > > It's just a helper for developers when adding new kvm_run fields, not > > a risk for anyone who wants to start allocating more pages for it. > > But by adding a BUILD_BUG_ON without explaining *why*, you're placing an > extra burden on someone that wants to increase the size of kvm->run, e.g. > it's not at all obvious from the changelog whether this patch is adding > the BUILD_BUG_ON purely because the code allocates memory for vcpu->run > via alloc_page(), or if there is some fundamental aspect of vcpu->run that > requires it to never span multiple pages. How about I add a comment above it? /* * Currently kvm_run only uses one physical page. Warn the develper * if kvm_run accidentaly grows more than that. */ BUILD_BUG_ON(...); Thanks, -- Peter Xu
