Re: [Xen-devel] Re: [PATCH] FPU LWP 6/8: create lazy and non-lazy FPU restore functions

[Wei Huang <wei.huang2@xxxxxxx>]

Hi Jan,

If we want to make LWP restore optional in vcpu_restore_fpu_eager(), we 
have to change vcpu_save_fpu() as well. Otherwise, the extended state 
will become inconsistent for non-LWP VCPUs (because save and restore is 
asymmetric). There are two approaches:
1. In vcpu_save_fpu(), clean physical CPU's extended state for VCPU 
which is being scheduled in. This prevents messy states from causing 
problems. The disadvantage is the cleaning cost, which would out-weight 
the benefits.
2. Add a new variable in VCPU to track whether nonlazy state is dirty. I 
think this is better. See the attached file.
Let me know if it is what you want. After that, I will re-spin the patches.

Thanks,
-Wei


On 05/05/2011 02:13 AM, Jan Beulich wrote:
> > > > On 04.05.11 at 18:33, Wei Huang<wei.huang2@xxxxxxx>  wrote:
> > Checking whether there is a non-lazy state to save is architectural
> > specific and very messy. For instance, we need to read LWP_CBADDR to
> > confirm LWP's dirty state. This MSR is AMD specific and we don't want to
> > add it here. Plus reading data from LWP_CBADDR MSR might be as expensive
> > as clts/stts.
> >
> > My previous email showed that the overhead with LWP is around 1%-2% of
> > __context_switch(). For non lwp-capable CPU, this overhead should be
> > much smaller (only clts and stts) because xfeature_mask[LWP] is 0.
> I wasn't talking about determining whether LWP state is dirty, but
> much rather about LWP not being in use at all.
>
> > Yes, clts() and stts() don't have to called every time. How about this one?
> >
> > /* Restore FPU state whenever VCPU is schduled in. */
> > void vcpu_restore_fpu_eager(struct vcpu *v)
> > {
> >       ASSERT(!is_idle_vcpu(v));
> >
> >
> >       /* save the nonlazy extended state which is not tracked by CR0.TS bit */
> >       if ( xsave_enabled(v) )
> >       {
> >           /* Avoid recursion */
> >           clts();
> >           fpu_xrstor(v, XSTATE_NONLAZY);
> >           stts();
> >       }
> That's certainly better, but I'd still like to see the xsave_enabled()
> check to be replaced by some form of lwp_enabled() or
> lazy_xsave_needed() or some such (which will at once exclude all
> pv guests until you care to add support for them).
>
> Jan
>
>
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Attachment: nonlazy_dirty.txt
Description: nonlazy_dirty.txt

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