From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [PATCH 0/7] Consolidate vcpu ioctl locking Date: Sun, 16 May 2010 12:09:58 +0300 Message-ID: <4BEFB666.50107@redhat.com> References: <1273749459-622-1-git-send-email-avi@redhat.com> <4BEBEA25.8080309@redhat.com> <4BEBEA7E.80202@redhat.com> <4BEBEAAE.9030502@redhat.com> <24423079-CDE0-4DEA-BC73-3B6976BE0CA6@suse.de> <4BEBF0BF.2020404@redhat.com> <4BEE3C56.2070007@redhat.com> <4BEE544B.50405@redhat.com> <20442124-2400-4273-A256-6846017D3141@suse.de> <4BEEDA37.2080209@redhat.com> <6BE91F3A-C60C-47C0-9EA4-E5F5971B09C2@suse.de> <4BEFAB6D.9000904@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Marcelo Tosatti , "kvm-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" , "kvm-ia64-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" , "kvm-ppc-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" , "carsteno-tA70FqPdS9bQT0dZR+AlfA@public.gmane.org" To: Alexander Graf Return-path: In-Reply-To: Sender: kvm-ppc-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-Id: kvm.vger.kernel.org On 05/16/2010 12:01 PM, Alexander Graf wrote: > >> That's what the world looked like in 2006. >> >> We could change it, but there's not much point, since having the local apic in the kernel is pretty much a requirement for reasonable performance. >> > Well, I'm not convinced yet that's the case for PPC as well. The timer is in-cpu anyways and I don't see why IPIs should be slow with a userspace pic - if we keep the overhead low. > If it's at all possible keep the mpic out. I am _not_ advocating pushing ppc's mpic into the kernel. > So let me think this through. With remote interrupt injection we have. > > * thread 1 does vcpu_run > * thread 2 triggers KVM_INTERRUPT on fd > * thread 2 signals thread 1 so we're sure the interrupt gets injected > * thread 1 exits into qemu > This doesn't seem necessary. The kernel can own the interrupt line, so it remembers it from the last KVM_INTERRUPT. > * thread 1 goes back into the vcpu, triggering an interrupt > > Without we have: > > * thread 1 does vcpu_run > * thread 2 wants to trigger an an interrupt, sets the qemu internal bit > * thread 2 signals thread 1 so we're sure the interrupt gets processed > * thread 1 exits into qemu > * thread 1 triggers KVM_INTERRUPT on fd > * thread 1 goes into the vcpu > > So we don't really buy anything from doing the remote injection. Hrm. > Not if you make interrupt injection a lightweight exit. > What's somewhat striking me here though is - why do we need KVM_INTERRUPT when there's all those kvm_run fields? Can't we just do interrupt injection by setting run->trigger_interrupt? There's only a single "interrupt line" on the CPU anyways. That way we'd save the ioctl and get rid of the locking problem altogether. > That's what x86 does. However, it's synchronous. -- error compiling committee.c: too many arguments to function