Sean Christopherson <sean.j.christopherson@xxxxxxxxx> writes: > +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? (Sorry for the delayed reply, was traveling last week) When KVM runs as an L1 on top of Hyper-V it uses eVMCS by default and eVMCSv1 doesn't support PML. I've also just checked Hyper-V 2019 and it hides SECONDARY_EXEC_ENABLE_PML from guests (this was expected). > > 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'd vote for (2): while this will effectively be kvm_flush_remote_tlbs() for now, we may think of something smarter in the future (e.g. PV interface for KVM-on-KVM). > > 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. > -- Vitaly
