linux-kernel - Re: [PATCH 11/11] KVM/MMU: Flush tlb in the kvm_age

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Message-ID: <d434edb8-5f09-7331-c89d-9b78907e896a@redhat.com>
Date:   Mon, 7 Jan 2019 17:31:26 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Tianyu Lan <lantianyu1986@...il.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Lan Tianyu <Tianyu.Lan@...rosoft.com>,
        Radim Krcmar <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, bp@...en8.de,
        "H. Peter Anvin" <hpa@...or.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        kvm <kvm@...r.kernel.org>,
        "linux-kernel@...r kernel org" <linux-kernel@...r.kernel.org>,
        michael.h.kelley@...rosoft.com, kys@...rosoft.com,
        vkuznets@...hat.com, linux@...linux.org.uk
Subject: Re: [PATCH 11/11] KVM/MMU: Flush tlb in the kvm_age_rmapp()

On 07/01/19 04:42, Tianyu Lan wrote:
>> I'm assuming you're
>> clearing young to avoid the flush in kvm_mmu_notifier_clear_flush_young(),
>> but keeping that flush is silly since it will never be invoked.  Just
>> squash this patch with patch 10/11 so that you can remove the unnecessary
>> flush in kvm_mmu_notifier_clear_flush_young() and preserve young.
>>
> The platform may provide tlb flush with address range as granularity. My changes
> are to use range flush when it's available. kvm_mmu_notifier_clear_flush_young()
> is common function for all platforms and most platforms still need the
> flush in the
> kvm_mmu_notifier_clear_flush_young(). I think it's better to separate
> flush request and
> "young" from return value of kvm_age_hva(). New flush parameter I
> added in the patch 10
> can be changed to a pointer and kvm_age_hva() can use it to return
> flush request.

There are two possibilities:

- pass a "bool *flush".  If NULL, kvm_age_hva should not flush.  If not
NULL, kvm_age_hva should receive a true *flush, and should change it to
false if kvm_age_hva takes care of the flush

- pass a "bool flush".  In patch 10, change all kvm_age_hva
implementation to do the flush if they return 1.

I think I prefer the latter, in this case the small code duplication is
offset by a simpler API.

Paolo