From mboxrd@z Thu Jan 1 00:00:00 1970 From: Sean Christopherson Date: Fri, 07 Feb 2020 19:45:32 +0000 Subject: Re: [PATCH v5 15/19] KVM: Provide common implementation for generic dirty log functions Message-Id: <20200207194532.GK2401@linux.intel.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> In-Reply-To: <20200206214106.GG700495@xz-x1> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Peter Xu , Vitaly Kuznetsov Cc: Paolo Bonzini , Paul Mackerras , Christian Borntraeger , Janosch Frank , David Hildenbrand , Cornelia Huck , Vitaly Kuznetsov , 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 =?iso-8859-1?Q?Mathieu-Daud=E9?= +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.