From mboxrd@z Thu Jan 1 00:00:00 1970 From: Vitaly Kuznetsov Date: Mon, 17 Feb 2020 15:35:05 +0000 Subject: Re: [PATCH v5 15/19] KVM: Provide common implementation for generic dirty log functions Message-Id: <87v9o59qhi.fsf@vitty.brq.redhat.com> List-Id: References: <20200121223157.15263-1-sean.j.christopherson@intel.com> <20200121223157.15263-16-sean.j.christopherson@intel.com> <20200206200200.GC700495@xz-x1> <20200206212120.GF13067@linux.intel.com> <20200206214106.GG700495@xz-x1> <20200207194532.GK2401@linux.intel.com> In-Reply-To: <20200207194532.GK2401@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Sean Christopherson , Peter Xu Cc: Paolo Bonzini , Paul Mackerras , Christian Borntraeger , Janosch Frank , David Hildenbrand , Cornelia Huck , Wanpeng Li , Jim Mattson , Joerg Roedel , Marc Zyngier , James Morse , Julien Thierry , Suzuki K Poulose , linux-mips@vger.kernel.org, kvm@vger.kernel.org, kvm-ppc@vger.kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, linux-kernel@vger.kernel.org, Christoffer Dall , Philippe =?utf-8?Q?Mathieu-Daud=C3=A9?= Sean Christopherson 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