Messages in this thread | | | Date | Thu, 26 Jan 2017 16:16:42 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH 5/7] x86/fpu: Change fpu->fpregs_active users to fpu->fpstate_active |
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* Rik van Riel <riel@redhat.com> wrote:
> On Thu, 2017-01-26 at 12:26 +0100, Ingo Molnar wrote: > > We want to simplify the FPU state machine by eliminating fpu- > > >fpregs_active, > > and we can do that because the two state flags (::fpregs_active and > > ::fpstate_active) are set essentially together. > > > > The old lazy FPU switching code used to make a distinction - but > > there's > > no lazy switching code anymore, we always switch in an 'eager' > > fashion. > > I've been working for a while now to fix that for > KVM VCPU threads. > > Currently when we switch to a VCPU thread, we first > load that thread's userspace FPU context, and then > soon after we save that, and load the guest side FPU > context. > > When a VCPU thread goes idle, we also go through > two FPU context transitions. > > In order to skip the unnecessary FPU context switches > for VCPU threads, I have been relying on separate > fpstate_active and fpregs_active states. > > Do you have any ideas on how I could implement that > kind of change without separate fpstate_active and > fpregs_active states?
So the vCPU threads have host side FPU (user-space) state - whatever FPU state Qemu has?
One solution to that overhead, without complicating the FPU state machine in any way, would be to add a facility to drop/reacquire that FPU state.
That should automatically result in zero FPU state switching AFAICS: kernel threads don't do FPU state switching either.
The vCPU threads sometimes do return to user-space, when they get some deep exception that needs to be handled by Qemu, right? This aspect shouldn't be a big problem either, because the regular calling convention is to call (synchronous) system calls without holding FPU state, right?
I.e. the vCPU /dev/kvm ioctl() could drop/re-map the FPU state with very little overhead (i.e. no full save/restore required in that code path either), when it enters/exits vCPU mode.
Thanks,
Ingo
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