From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [PATCH RFC dontapply] kvm_para: add mmio word store hypercall Date: Mon, 26 Mar 2012 12:16:14 +0200 Message-ID: <4F7041EE.10305@redhat.com> References: <20120325220518.GA27879@redhat.com> <4F703536.3040904@redhat.com> <20120326100829.GA14506@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: Joerg Roedel , Marcelo Tosatti , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , kvm@vger.kernel.org, linux-kernel@vger.kernel.org To: "Michael S. Tsirkin" Return-path: In-Reply-To: <20120326100829.GA14506@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On 03/26/2012 12:08 PM, Michael S. Tsirkin wrote: > > > > + gpa = hc_gpa(vcpu, a1, a2); > > > + if (!write_mmio(vcpu, gpa, 2, &a0) && run) { > > > > What's this && run thing? > > I'm not sure - copied this from another other place in emulation: > arch/x86/kvm/x86.c:4953: if (!write_mmio(vcpu, gpa, 2, &a0) && run) > > I assumed there's some way to trigger emulation while VCPU does not run. > No? Not the way you initialize run above. > > > > > > + run->exit_reason = KVM_EXIT_MMIO; > > > + run->mmio.phys_addr = gpa; > > > + memcpy(run->mmio.data, &a0, 2); > > > + run->mmio.len = 2; > > > + run->mmio.is_write = 1; > > > + r = 0; > > > + } > > > + goto noret; > > > > What if the address is in RAM? > > Note the guest can't tell if a piece of memory is direct mapped or > > implemented as mmio. > > True but doing hypercalls for memory which can be > mapped directly is bad for performance - it's > the reverse of what we are trying to do here. It's bad, but the guest can't tell. Suppose someone implements virtio in hardware and we pass it through to a guest. It should continue working, no? > The intent is to use this for virtio where we can explicitly let the > guest know whether using a hypercall is safe. > > Acceptable? What do you suggest? It's iffy. What's the performance gain from this thing? -- error compiling committee.c: too many arguments to function