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SubjectRe: [PATCH v2 3/7] KVM: Add paravirt kvm_flush_tlb_others
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Hi Marcelo,

Thanks for the review.

On Tue, 12 Jun 2012 20:02:18 -0300, Marcelo Tosatti <mtosatti@redhat.com> wrote:
> On Mon, Jun 04, 2012 at 10:37:24AM +0530, Nikunj A. Dadhania wrote:
> > flush_tlb_others_ipi depends on lot of statics in tlb.c. Replicated
> > the flush_tlb_others_ipi as kvm_flush_tlb_others to further adapt to
> > paravirtualization.
> >
> > Use the vcpu state information inside the kvm_flush_tlb_others to
> > avoid sending ipi to pre-empted vcpus.
> >
> > * Do not send ipi's to offline vcpus and set flush_on_enter flag
> > * For online vcpus: Wait for them to clear the flag
> >
> > The approach was discussed here: https://lkml.org/lkml/2012/2/20/157
> >
> > Suggested-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> > Signed-off-by: Nikunj A. Dadhania <nikunj@linux.vnet.ibm.com>
>
> Why not reintroduce the hypercall to flush TLBs?
Sure, I will get a version with this.

> No waiting, no entry/exit trickery.
>
Are you also suggesting to get rid of vcpu-running information?
We would atleast need this to raise a flushTLB hypercall on the sleeping
vcpu.

> This is half-way-there paravirt with all the downsides.
Some more details on what are the downsides would help us reach to a
better solution.

> Even though the guest running information might be useful in other
> cases.
>
Yes, that was one of the things on the mind.

> > Pseudo Algo:
> >
> > Write()
> > ======
> >
> > guest_exit()
> > flush_on_enter[i]=0;
> > running[i] = 0;
> >
> > guest_enter()
> > running[i] = 1;
> > smp_mb();
> > if(flush_on_enter[i]) {
> > tlb_flush()
> > flush_on_enter[i]=0;
> > }
> >
> >
> > Read()
> > ======
> >
> > GUEST KVM-HV
> >
> > f->flushcpumask = cpumask - me;
> >
> > again:
> > for_each_cpu(i, f->flushmask) {
> >
> > if (!running[i]) {
> > case 1:
> >
> > running[n]=1
> >
> > (cpuN does not see
> > flush_on_enter set,
> > guest later finds it
> > running and sends ipi,
> > we are fine here, need
> > to clear the flag on
> > guest_exit)
> >
> > flush_on_enter[i] = 1;
> > case2:
> >
> > running[n]=1
> > (cpuN - will see flush
> > on enter and an IPI as
> > well - addressed in patch-4)
> >
> > if (!running[i])
> > cpu_clear(f->flushmask); All is well, vm_enter
> > will do the fixup
> > }
> > case 3:
> > running[n] = 0;
> >
> > (cpuN went to sleep,
> > we saw it as awake,
> > ipi sent, but wait
> > will break without
> > zero_mask and goto
> > again will take care)
> >
> > }
> > send_ipi(f->flushmask)
> >
> > wait_a_while_for_zero_mask();
> >
> > if (!zero_mask)
> > goto again;
> > ---
> > arch/x86/include/asm/kvm_para.h | 3 +-
> > arch/x86/include/asm/tlbflush.h | 9 ++++++
> > arch/x86/kernel/kvm.c | 1 +
> > arch/x86/kvm/x86.c | 14 ++++++++-
> > arch/x86/mm/tlb.c | 61 +++++++++++++++++++++++++++++++++++++++
> > 5 files changed, 86 insertions(+), 2 deletions(-)
> >
>
>
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