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(). 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.
