On Mon, 2022-05-16 at 15:14 +0000, Sean Christopherson wrote: > On Mon, May 16, 2022, Maxim Levitsky wrote: > > On Mon, 2022-05-16 at 14:08 +0000, Sean Christopherson wrote: > > > On Mon, May 16, 2022, Maxim Levitsky wrote: > > > > On Wed, 2022-05-11 at 21:54 +0200, Uros Bizjak wrote: > > > > > On Wed, May 11, 2022 at 6:04 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > > > > > > On Wed, May 11, 2022, Uros Bizjak wrote: > > > > > > > On Wed, May 11, 2022 at 9:54 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > > > > > > > > Still, does 32bit actually support that stuff? > > > > > > > > > > > > > > Unfortunately, it does: > > > > > > > > > > > > > > kvm-intel-y += vmx/vmx.o vmx/vmenter.o vmx/pmu_intel.o vmx/vmcs12.o \ > > > > > > > vmx/evmcs.o vmx/nested.o vmx/posted_intr.o > > > > > > > > > > > > > > And when existing cmpxchg64 is substituted with cmpxchg, the > > > > > > > compilation dies for 32bits with: > > > > > > > > > > > > ... > > > > > > > > > > > > > > Anyway, your patch looks about right, but I find it *really* hard to > > > > > > > > care about 32bit code these days. > > > > > > > > > > > > > > Thanks, this is also my sentiment, but I hope the patch will enable > > > > > > > better code and perhaps ease similar situation I have had elsewhere. > > > > > > > > > > > > IMO, if we merge this it should be solely on the benefits to 64-bit code. Yes, > > > > > > KVM still supports 32-bit kernels, but I'm fairly certain the only people that > > > > > > run 32-bit KVM are KVM developers. 32-bit KVM has been completely broken for > > > > > > multiple releases at least once, maybe twice, and no one ever complained. > > > > > > > > > > Yes, the idea was to improve cmpxchg64 with the implementation of > > > > > try_cmpxchg64 for 64bit targets. However, the issue with 32bit targets > > > > > stood in the way, so the effort with 32-bit implementation was mainly > > > > > to unblock progression for 64-bit targets. > > > > > > > > Would that allow tdp mmu to work on 32 bit? > > > > > > From a purely technical perspective, there's nothing that prevents enabling the > > > TDP MMU on 32-bit kernels. The TDP MMU is 64-bit only to simplify the implementation > > > and to reduce the maintenance and validation costs. > > > > I understand exactly that, so the question, will this patch help make the tdp > > mmu work transparently on 32 bit kernels? I heard that 64 bit cmpxchg was > > one of the main reasons that it is 64 bit only. > > I don't think it moves the needled much, e.g. non-atomic 64-bit accesses are still > problematic, and we'd have to update the TDP MMU to deal with PAE paging (thanks > NPT). All those problems are solvable, it's purely a matter of the ongoing costs > to solve them. > > > I am asking because there was some talk to eliminate the direct mode from the > > legacy non tdp mmu, which would simplify its code by a lot, but then it will > > make 32 bit kernel fail back to shadowing mmu. > > Simplify which code? Between the nonpaging code and direct shadow pages in > indirect MMUs, the vast majority of the "direct" support in the legacy MMU needs > to be kept even if TDP support is dropped. And the really nasty stuff, e.g. PAE > roots, would need to be carried over to the TDP MMU. > I guess this makes sense. I haven't researched the code well enough to know the exact answer. I was just curious if this patch makes any difference :) Thanks! Best regards, Maxim Levitsky
