On 09/10/2015 15:39, Denis V. Lunev wrote:> From: Andrey Smetanin <asmetanin at virtuozzo.com> > > 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. > > Signed-off-by: Andrey Smetanin <asmetanin at virtuozzo.com> > Reviewed-by: Roman Kagan <rkagan at virtiozzo.com> > Signed-off-by: Denis V. Lunev <den at openvz.org> > CC: Vitaly Kuznetsov <vkuznets at redhat.com> > CC: "K. Y. Srinivasan" <kys at microsoft.com> > CC: Gleb Natapov <gleb at kernel.org> > CC: Paolo Bonzini <pbonzini at redhat.com>Why is this exit necessary? Paolo
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.
On 09/10/2015 16:53, Roman Kagan wrote:>> > 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.Ok, thanks! Paolo