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From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH] KVM: x86/mmu: Avoid retpoline on ->page_fault() with TDP
Date: Fri, 07 Feb 2020 17:15:49 +0100	[thread overview]
Message-ID: <8736bmqsp6.fsf@vitty.brq.redhat.com> (raw)
In-Reply-To: <20200207155539.GC2401@linux.intel.com>

Sean Christopherson <sean.j.christopherson@intel.com> writes:

> On Fri, Feb 07, 2020 at 10:29:16AM +0100, Vitaly Kuznetsov wrote:
>> Sean Christopherson <sean.j.christopherson@intel.com> writes:
>> 
>> > Wrap calls to ->page_fault() with a small shim to directly invoke the
>> > TDP fault handler when the kernel is using retpolines and TDP is being
>> > used.  Denote the TDP fault handler by nullifying mmu->page_fault, and
>> > annotate the TDP path as likely to coerce the compiler into preferring
>> > the TDP path.
>> >
>> > Rename tdp_page_fault() to kvm_tdp_page_fault() as it's exposed outside
>> > of mmu.c to allow inlining the shim.
>> >
>> > Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
>> > ---
>> 
>> Out of pure curiosity, if we do something like
>> 
>> if (vcpu->arch.mmu->page_fault == tdp_page_fault)
>>     tdp_page_fault(...)
>> else if (vcpu->arch.mmu->page_fault == nonpaging_page_fault)
>>    nonpaging_page_fault(...)
>> ...
>> 
>> we also defeat the retpoline, right?
>
> Yep.
>
>> Should we use this technique ... everywhere? :-)
>
> It becomes a matter of weighing the maintenance cost and robustness against
> the performance benefits.  For the TDP case, amost no one (that cares about
> performance) uses shadow paging, the change is very explicit, tiny and
> isolated, and TDP page fault are a hot path, e.g. when booting the VM.
> I.e. low maintenance overhead, still robust, and IMO worth the shenanigans.
>
> The changes to VMX's VM-Exit handlers follow similar thinking: snipe off
> the exit handlers that are performance critical, but use a low maintenance
> implementation for the majority of handlers.
>
> There have been multiple attempts to add infrastructure to solve the
> maintenance and robustness problems[*], but AFAIK none of them have made
> their way upstream.
>
> [*] https://lwn.net/Articles/774743/
>

Oh I see, missed some of these discussion.

And I actualy forgot to say:

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>

as the patch itself looks good to me, I was just wondering about the
approach in general.

-- 
Vitaly


  reply	other threads:[~2020-02-07 16:16 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-06 22:14 Sean Christopherson
2020-02-07  9:29 ` Vitaly Kuznetsov
2020-02-07 15:55   ` Sean Christopherson
2020-02-07 16:15     ` Vitaly Kuznetsov [this message]
2020-02-12 11:55     ` Paolo Bonzini
2020-02-12 16:22       ` Sean Christopherson

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