From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Date: Sun, 16 May 2010 09:47:46 +0000 Subject: Re: [PATCH 0/7] Consolidate vcpu ioctl locking Message-Id: <4BEFBF42.6020208@redhat.com> List-Id: References: <1273749459-622-1-git-send-email-avi@redhat.com> In-Reply-To: <1273749459-622-1-git-send-email-avi@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: kvm-ia64@vger.kernel.org On 05/16/2010 12:35 PM, Alexander Graf wrote: > >> >>> 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. >> > It's not? With s/signals/IPIs/. > On signals we always exit to userspace, no? > Yes (if the signal isn't blocked). >>> * 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. >> > Please elaborate. > 1: vcpu_run 2: KVM_INTERRUPT 2k: sets flag, if msr.ee IPIs 1 or wakes up 1 if halted 1k: notices flag, if msr.ee injects interrupt ... 1g: acks 1k: forwards ack to userspace 1: completes interrupt >>> 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. >> > For everyone except for the vcpu thread executing the interrupt, it's asynchronous, right? For everyone other than the vcpu thread, it's off limits. kvm_run is only read on KVM_RUN entries and written on KVM_RUN exits. > The same applies to an in-kernel pic. > The in-kernel pic doesn't use kvm_run. -- error compiling committee.c: too many arguments to function