On Fri, May 18, 2018 at 01:04:31PM -0300, Eduardo Habkost wrote: > CCing qemu-devel, as I'm now discussing userspace. > > On Thu, May 17, 2018 at 10:55:33PM +0300, Michael S. Tsirkin wrote: > > On Thu, May 17, 2018 at 03:46:58PM -0300, Eduardo Habkost wrote: > > > On Thu, May 17, 2018 at 05:54:24PM +0300, Michael S. Tsirkin wrote: > > > > HINTS_DEDICATED seems to be somewhat confusing: > > > > > > > > Guest doesn't really care whether it's the only task running on a host > > > > CPU as long as it's not preempted. > > > > > > > > And there are more reasons for Guest to be preempted than host CPU > > > > sharing, for example, with memory overcommit it can get preempted on a > > > > memory access, post copy migration can cause preemption, etc. > > > > > > > > Let's call it KVM_HINTS_REALTIME which seems to better > > > > match what guests expect. > > > > > > > > Also, the flag most be set on all vCPUs - current guests assume th. > > > > Note so in the documentation. > > > > > > > > Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx> > > > > --- > > > > Documentation/virtual/kvm/cpuid.txt | 6 +++--- > > > > arch/x86/include/uapi/asm/kvm_para.h | 2 +- > > > > arch/x86/kernel/kvm.c | 8 ++++---- > > > > 3 files changed, 8 insertions(+), 8 deletions(-) > > > > > > > > diff --git a/Documentation/virtual/kvm/cpuid.txt b/Documentation/virtual/kvm/cpuid.txt > > > > index d4f33eb8..ab022dc 100644 > > > > --- a/Documentation/virtual/kvm/cpuid.txt > > > > +++ b/Documentation/virtual/kvm/cpuid.txt > > > > @@ -72,8 +72,8 @@ KVM_FEATURE_CLOCKSOURCE_STABLE_BIT || 24 || host will warn if no guest-side > > > > > > > > flag || value || meaning > > > > ================================================================================== > > > > -KVM_HINTS_DEDICATED || 0 || guest checks this feature bit to > > > > - || || determine if there is vCPU pinning > > > > - || || and there is no vCPU over-commitment, > > > > +KVM_HINTS_REALTIME || 0 || guest checks this feature bit to > > > > + || || determine that vCPUs are never > > > > + || || preempted for an unlimited time, > > > > || || allowing optimizations > > > > > > My understanding of the original patch is that the intention is > > > to tell the guest that it is very unlikely to be preempted, > > > so it > > > can choose a more appropriate spinlock implementation. This > > > description implies that the guest will never be preempted, which > > > is much stronger guarantee. > > > > Note: > > > > ... for an unlimited time. > > Which still sounds like a stronger guarantee than the original > description. But: > > > > > > > > > Isn't this new description incompatible with existing usage of > > > the hint, which might include people who just use vCPU pinning > > > but no mlock? > > > > Without mlock you should always use pv spinlocks. > > > > Otherwise you risk blocking on a lock taken by > > a VCPU that is in turn blocked on IO, where the IO > > is not completing because CPU is being used up > > spinning. > > So the stronger guarantee seems necessary. > > Now what should host userspace do if the user is trying to run an > existing configuration where the CPUID hint was set but memory is > not pinned? > > -- > Eduardo As much as we'd like to be helpful and validate input, you need a real time host too. I'm not sure how we'd find out - I suggest we do not bother for now. -- MST
