From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [RFC] Handling VMEXITS with interrupts disabled? Date: Mon, 23 Apr 2007 10:02:44 +0300 Message-ID: <462C5A14.7050108@qumranet.com> References: <462C2345.6040507@codemonkey.ws> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: kvm-devel To: Anthony Liguori Return-path: In-Reply-To: <462C2345.6040507-rdkfGonbjUSkNkDKm+mE6A@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: kvm-devel-bounces-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org Errors-To: kvm-devel-bounces-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org List-Id: kvm.vger.kernel.org Anthony Liguori wrote: > Howdy, > > I had an idea for improving VMEXIT time by only saving/restoring > sys{enter,call,ret} MSRs when exiting from kernel space code since as > long as these instructions aren't executed on the host, everything > should be fine. > Yup. > This worked fine for MSR_IA32_SYSENTER* msrs, but not so much for the > ?TAR msrs. I have hooks in vcpu_{get,put} but I suspect we're making a > blocking call somewhere which is resulting in the scheduler being > invoked (while we still have the vcpu). This then results in generally > badness in the host. > You might compile the host with CONFIG_PREEMPT, which should get you a nice stacktrace after the "scheduling while atomic" message. > I'm still a little unclear about this though as I would think that if > this could happen, it would create problems with VT since KVM another > guest could run and overwrite the VMCS msr. > > I'm I interpreting this correctly? Does disabling preemption not > guarantee that you won't potentially drop to userspace in your critical > block? > This is only checked under CONFIG_PREEMPT, otherwise preempt_disable (and get_cpu()) are no-ops. I really need to enable CONFIG_PREEMPT myself... -- error compiling committee.c: too many arguments to function ------------------------------------------------------------------------- This SF.net email is sponsored by DB2 Express Download DB2 Express C - the FREE version of DB2 express and take control of your XML. No limits. Just data. Click to get it now. http://sourceforge.net/powerbar/db2/