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[34.168.104.7]) by smtp.gmail.com with ESMTPSA id d125-20020a623683000000b0056c0b472c09sm3022854pfa.211.2022.10.28.09.31.46 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 28 Oct 2022 09:31:46 -0700 (PDT) Date: Fri, 28 Oct 2022 16:31:42 +0000 From: Sean Christopherson To: Paolo Bonzini Cc: "Woodhouse, David" , linux-kernel@vger.kernel.org, kvm@vger.kernel.org, mhal@rbox.co Subject: Re: [PATCH 03/16] KVM: x86: set gfn-to-pfn cache length consistently with VM word size Message-ID: References: <20221027161849.2989332-1-pbonzini@redhat.com> <20221027161849.2989332-4-pbonzini@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Oct 28, 2022, Paolo Bonzini wrote: > On 10/27/22 19:22, Sean Christopherson wrote: > > On Thu, Oct 27, 2022, Paolo Bonzini wrote: > > > So, use the short size at activation time as well. This means > > > re-activating the cache if the guest requests the hypercall page > > > multiple times with different word sizes (this can happen when > > > kexec-ing, for example). > > > > I don't understand the motivation for allowing a conditionally valid GPA. I see > > a lot of complexity and sub-optimal error handling for a use case that no one > > cares about. Realistically, userspace is never going to provide a GPA that only > > works some of the time, because doing otherwise is just asking for a dead guest. > > We _should_ be following the Xen API, which does not even say that the > areas have to fit in a single page. Ah, I didn't realize these are hypercall => userspace => ioctl() paths. > In fact, even Linux's > > struct vcpu_register_runstate_memory_area area; > > area.addr.v = &per_cpu(xen_runstate, cpu); > if (HYPERVISOR_vcpu_op(VCPUOP_register_runstate_memory_area, > xen_vcpu_nr(cpu), &area)) > > could fail or not just depending on the linker's whims, if I'm not > very confused. > > Other data structures *do* have to fit in a page, but the runstate area > does not and it's exactly the one where the cache comes the most handy. > For this I'm going to wait for David to answer. > > That said, the whole gpc API is really messy No argument there. > and needs to be cleaned up beyond what this series does. For example, > > read_lock_irqsave(&gpc->lock, flags); > while (!kvm_gfn_to_pfn_cache_check(v->kvm, gpc, gpc->gpa, > sizeof(x))) { > read_unlock_irqrestore(&gpc->lock, flags); > > if (kvm_gfn_to_pfn_cache_refresh(v->kvm, gpc, gpc->gpa, sizeof(x))) > return; > > read_lock_irqsave(&gpc->lock, flags); > } > ... > read_unlock_irqrestore(&gpc->lock, flags); > > should really be simplified to > > khva = kvm_gpc_lock(gpc); > if (IS_ERR(khva)) > return; > ... > kvm_gpc_unlock(gpc); > > Only the special preempt-notifier cases would have to use check/refresh > by hand. If needed they could even pass whatever length they want to > __kvm_gpc_refresh with, explicit marking that it's a here-be-dragons __API. > > Also because we're using the gpc from non-preemptible regions the rwlock > critical sections should be enclosed in preempt_disable()/preempt_enable(). > Fortunately they're pretty small. > > For now I think the best course of action is to quickly get the bugfix > patches to Linus, and for 6.2 drop this one but otherwise keep the length > in kvm_gpc_activate(). Works for me.