From mboxrd@z Thu Jan 1 00:00:00 1970 From: Roman Kagan Subject: Re: [PATCH 2/2] kvm/x86: Hyper-V kvm exit Date: Fri, 9 Oct 2015 17:53:59 +0300 Message-ID: <20151009145358.GT27239__24388.7302269743$1444649887$gmane$org@rkaganb.sw.ru> References: <1444397988-20167-1-git-send-email-den@openvz.org> <1444397988-20167-3-git-send-email-den@openvz.org> <5617D20B.1080402@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <5617D20B.1080402@redhat.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org To: Paolo Bonzini Cc: Gleb Natapov , qemu-devel@nongnu.org, virtualization@lists.linux-foundation.org, Andrey Smetanin , "Denis V. Lunev" , Vitaly Kuznetsov List-Id: virtualization@lists.linuxfoundation.org On Fri, Oct 09, 2015 at 04:41:15PM +0200, Paolo Bonzini wrote: > On 09/10/2015 15:39, Denis V. Lunev wrote: > > A new vcpu exit is introduced to notify the userspace of the > > changes in Hyper-V synic configuraion triggered by guest writing to the > > corresponding MSRs. > > Why is this exit necessary? The guest writes to synic-related MSRs and that should take "immediate" effect. E.g. it may decide to disable or relocate the message page by writing to SIMP MSR. The host is then supposed to stop accessing the old message page before the vCPU proceeds to the next instruction. Hence the exit, to allow the userspace to react accordingly before reentering the guest. Roman.