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

From: Nadav Amit <namit@vmware.com>
To: Andy Lutomirski <luto@kernel.org>
Cc: Dave Hansen <dave.hansen@intel.com>,
	Peter Zijlstra <peterz@infradead.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	the arch/x86 maintainers <x86@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>
Subject: Re: [PATCH 6/9] KVM: x86: Provide paravirtualized flush_tlb_multi()
Date: Wed, 26 Jun 2019 03:41:37 +0000	[thread overview]
Message-ID: <35755C67-E8EB-48C3-8343-BB9ABEB4E32C@vmware.com> (raw)
In-Reply-To: <CALCETrWcUWw8ep-n6RaOeojnL924xOM7g7eb9g=3DRwOHQAgnA@mail.gmail.com>

> On Jun 25, 2019, at 8:35 PM, Andy Lutomirski <luto@kernel.org> wrote:
> 
> 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.

What do you mean by “do a remote flush”?