Sean Christopherson <sean.j.christopherson@xxxxxxxxx> writes: > On Fri, Feb 07, 2020 at 07:53:34PM -0500, Peter Xu wrote: >> On Fri, Feb 07, 2020 at 04:42:33PM -0800, Sean Christopherson wrote: >> > On Fri, Feb 07, 2020 at 07:18:32PM -0500, Peter Xu wrote: >> > > On Fri, Feb 07, 2020 at 11:45:32AM -0800, Sean Christopherson wrote: >> > > > +Vitaly for HyperV >> > > > >> > > > On Thu, Feb 06, 2020 at 04:41:06PM -0500, Peter Xu wrote: >> > > > > On Thu, Feb 06, 2020 at 01:21:20PM -0800, Sean Christopherson wrote: >> > > > > > On Thu, Feb 06, 2020 at 03:02:00PM -0500, Peter Xu wrote: >> > > > > > > But that matters to this patch because if MIPS can use >> > > > > > > kvm_flush_remote_tlbs(), then we probably don't need this >> > > > > > > arch-specific hook any more and we can directly call >> > > > > > > kvm_flush_remote_tlbs() after sync dirty log when flush==true. >> > > > > > >> > > > > > Ya, the asid_flush_mask in kvm_vz_flush_shadow_all() is the only thing >> > > > > > that prevents calling kvm_flush_remote_tlbs() directly, but I have no >> > > > > > clue as to the important of that code. >> > > > > >> > > > > As said above I think the x86 lockdep is really not necessary, then >> > > > > considering MIPS could be the only one that will use the new hook >> > > > > introduced in this patch... Shall we figure that out first? >> > > > >> > > > So I prepped a follow-up patch to make kvm_arch_dirty_log_tlb_flush() a >> > > > MIPS-only hook and use kvm_flush_remote_tlbs() directly for arm and x86, >> > > > but then I realized x86 *has* a hook to do a precise remote TLB flush. >> > > > There's even an existing kvm_flush_remote_tlbs_with_address() call on a >> > > > memslot, i.e. this exact scenario. So arguably, x86 should be using the >> > > > more precise flush and should keep kvm_arch_dirty_log_tlb_flush(). >> > > > >> > > > But, the hook is only used when KVM is running as an L1 on top of HyperV, >> > > > and I assume dirty logging isn't used much, if at all, for L1 KVM on >> > > > HyperV? >> > > > >> > > > I see three options: >> > > > >> > > > 1. Make kvm_arch_dirty_log_tlb_flush() MIPS-only and call >> > > > kvm_flush_remote_tlbs() directly for arm and x86. Add comments to >> > > > explain when an arch should implement kvm_arch_dirty_log_tlb_flush(). >> > > > >> > > > 2. Change x86 to use kvm_flush_remote_tlbs_with_address() when flushing >> > > > a memslot after the dirty log is grabbed by userspace. >> > > > >> > > > 3. Keep the resulting code as is, but add a comment in x86's >> > > > kvm_arch_dirty_log_tlb_flush() to explain why it uses >> > > > kvm_flush_remote_tlbs() instead of the with_address() variant. >> > > > >> > > > I strongly prefer to (2) or (3), but I'll defer to Vitaly as to which of >> > > > those is preferable. >> > > > >> > > > I don't like (1) because (a) it requires more lines code (well comments), >> > > > to explain why kvm_flush_remote_tlbs() is the default, and (b) it would >> > > > require even more comments, which would be x86-specific in generic KVM, >> > > > to explain why x86 doesn't use its with_address() flush, or we'd lost that >> > > > info altogether. >> > > > >> > > >> > > I proposed the 4th solution here: >> > > >> > > https://lore.kernel.org/kvm/20200207223520.735523-1-peterx@xxxxxxxxxx/ >> > > >> > > I'm not sure whether that's acceptable, but if it can, then we can >> > > drop the kvm_arch_dirty_log_tlb_flush() hook, or even move on to >> > > per-slot tlb flushing. >> > >> > This effectively is per-slot TLB flushing, it just has a different name. >> > I.e. s/kvm_arch_dirty_log_tlb_flush/kvm_arch_flush_remote_tlbs_memslot. >> > I'm not opposed to that name change. And on second and third glance, I >> > probably prefer it. That would more or less follow the naming of >> > kvm_arch_flush_shadow_all() and kvm_arch_flush_shadow_memslot(). >> >> Note that the major point of the above patchset is not about doing tlb >> flush per-memslot or globally. It's more about whether we can provide >> a common entrance for TLB flushing. Say, after that series, we should >> be able to flush TLB on all archs (majorly, including MIPS) as: >> >> kvm_flush_remote_tlbs(kvm); >> >> And with the same idea we can also introduce the ranged version. >> >> > >> > I don't want to go straight to kvm_arch_flush_remote_tlb_with_address() >> > because that loses the important distinction (on x86) that slots_lock is >> > expected to be held. >> >> Sorry I'm still puzzled on why that lockdep is so important and >> special for x86... For example, what if we move that lockdep to the >> callers of the kvm_arch_dirty_log_tlb_flush() calls so it protects >> even more arch (where we do get/clear dirty log)? IMHO the callers >> must be with the slots_lock held anyways no matter for x86 or not. > > > Following the breadcrumbs leads to the comment in > kvm_mmu_slot_remove_write_access(), which says: > > /* > * kvm_mmu_slot_remove_write_access() and kvm_vm_ioctl_get_dirty_log() > * which do tlb flush out of mmu-lock should be serialized by > * kvm->slots_lock otherwise tlb flush would be missed. > */ > > I.e. write-protecting a memslot and grabbing the dirty log for the memslot > need to be serialized. It's quite obvious *now* that get_dirty_log() holds > slots_lock, but the purpose of lockdep assertions isn't just to verify the > current functionality, it's to help ensure the correctness for future code > and to document assumptions in the code. > > Digging deeper, there are four functions, all related to dirty logging, in > the x86 mmu that basically open code what x86's > kvm_arch_flush_remote_tlbs_memslot() would look like if it uses the range > based flushing. > > Unless it's functionally incorrect (Vitaly?), going with option (2) and > naming the hook kvm_arch_flush_remote_tlbs_memslot() seems like the obvious > choice, e.g. the final cleanup gives this diff stat: (I apologize again for not replying in time) I think this is a valid approach and your option (2) would also be my choice. I also don't think there's going to be a problem when (if) Hyper-V adds support for PML (eVMCSv2?). > > arch/x86/kvm/mmu/mmu.c | 34 +++++++++------------------------- > 1 file changed, 9 insertions(+), 25 deletions(-) > Looks nice :-) -- Vitaly
