linux-kernel - Re: [PATCH v4 2/4] KVM: X86: Add paravirt remote TLB flush

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Message-ID: <20171114075636.rzozbjysqjpomguw@hirez.programming.kicks-ass.net>
Date:   Tue, 14 Nov 2017 08:56:36 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>, Paolo Bonzini <pbonzini@...hat.com>,
        Radim Kr??m???? <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH v4 2/4] KVM: X86: Add paravirt remote TLB flush

On Tue, Nov 14, 2017 at 02:28:56PM +0800, Wanpeng Li wrote:
> >  - have the TLB invalidate handler do something like:
> >
> >    state = READ_ONCE(src->preempted);
> >    if (!(state & KVM_VCPU_IPI_PENDING))
> >            return;
> >
> >    local_flush_tlb();
> >
> >    do {
> >    } while (!try_cmpxchg(&src->preempted, &state,
> >                          state & ~KVM_VCPU_IPI_PENDING));
> 
> There are a lot of cases handled by flush_tlb_func_remote() ->
> flush_tlb_function_common(), so I'm afraid to have hole.

Sure, just fix the handler to do what must be done. The above was merely
a sketch. The important part is to only clear IPI_PENDING after we do
the actual flushing, since the caller is waiting for that bit.


So flush_tlb_others() has two callers:

 - arch_tlbbatch_flush() -- info::end = TLB_FLUSH_ALL
 - flush_tlb_mm_range()  -- info::mm = mm

native_flush_tlb_others() does smp_call_function_many(
.func=flush_tlb_func_remote) which in turn calls flush_tlb_func_common(
.local=false, .reason=TLB_REMOTE_SHOOTDOWN).


So something like:

	struct flush_tlb_info info = {
		.start = 0,
		.end = TLB_FLUSH_ALL,
	};

	flush_tlb_func_common(&info, false, TLB_REMOTE_SHOOTDOWN);

should work for the new IPI. It 'upgrades' all ranges to full flushes,
but meh.