Re: [PATCH 6/9] KVM: x86: Provide paravirtualized flush_tlb_multi()

[Andy Lutomirski <luto@kernel.org>]

On Tue, Jun 25, 2019 at 7:39 PM Nadav Amit <namit@vmware.com> wrote:
>
> > On Jun 25, 2019, at 2:40 PM, Dave Hansen <dave.hansen@intel.com> wrote:
> >
> > On 6/12/19 11:48 PM, Nadav Amit wrote:
> >> Support the new interface of flush_tlb_multi, which also flushes the
> >> local CPU's TLB, instead of flush_tlb_others that does not. This
> >> interface is more performant since it parallelize remote and local TLB
> >> flushes.
> >>
> >> The actual implementation of flush_tlb_multi() is almost identical to
> >> that of flush_tlb_others().
> >
> > This confused me a bit.  I thought we didn't support paravirtualized
> > flush_tlb_multi() from reading earlier in the series.
> >
> > But, it seems like that might be Xen-only and doesn't apply to KVM and
> > paravirtualized KVM has no problem supporting flush_tlb_multi().  Is
> > that right?  It might be good to include some of that background in the
> > changelog to set the context.
>
> I’ll try to improve the change-logs a bit. There is no inherent reason for
> PV TLB-flushers not to implement their own flush_tlb_multi(). It is left
> for future work, and here are some reasons:
>
> 1. Hyper-V/Xen TLB-flushing code is not very simple
> 2. I don’t have a proper setup
> 3. I am lazy
>

In the long run, I think that we're going to want a way for one CPU to
do a remote flush and then, with appropriate locking, update the
tlb_gen fields for the remote CPU.  Getting this right may be a bit
nontrivial.