Sean Christopherson <sean.j.christopherson@xxxxxxxxx> writes: > On Fri, Feb 07, 2020 at 10:29:16AM +0100, Vitaly Kuznetsov wrote: >> Sean Christopherson <sean.j.christopherson@xxxxxxxxx> writes: >> >> > Wrap calls to ->page_fault() with a small shim to directly invoke the >> > TDP fault handler when the kernel is using retpolines and TDP is being >> > used. Denote the TDP fault handler by nullifying mmu->page_fault, and >> > annotate the TDP path as likely to coerce the compiler into preferring >> > the TDP path. >> > >> > Rename tdp_page_fault() to kvm_tdp_page_fault() as it's exposed outside >> > of mmu.c to allow inlining the shim. >> > >> > Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx> >> > --- >> >> Out of pure curiosity, if we do something like >> >> if (vcpu->arch.mmu->page_fault == tdp_page_fault) >> tdp_page_fault(...) >> else if (vcpu->arch.mmu->page_fault == nonpaging_page_fault) >> nonpaging_page_fault(...) >> ... >> >> we also defeat the retpoline, right? > > Yep. > >> Should we use this technique ... everywhere? :-) > > It becomes a matter of weighing the maintenance cost and robustness against > the performance benefits. For the TDP case, amost no one (that cares about > performance) uses shadow paging, the change is very explicit, tiny and > isolated, and TDP page fault are a hot path, e.g. when booting the VM. > I.e. low maintenance overhead, still robust, and IMO worth the shenanigans. > > The changes to VMX's VM-Exit handlers follow similar thinking: snipe off > the exit handlers that are performance critical, but use a low maintenance > implementation for the majority of handlers. > > There have been multiple attempts to add infrastructure to solve the > maintenance and robustness problems[*], but AFAIK none of them have made > their way upstream. > > [*] https://lwn.net/Articles/774743/ > Oh I see, missed some of these discussion. And I actualy forgot to say: Reviewed-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> as the patch itself looks good to me, I was just wondering about the approach in general. -- Vitaly
