linux-kernel - Re: [PATCH] kvm x86/mmu: use KVM_REQ_MMU

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Message-ID: <87y2ltx6gl.fsf@vitty.brq.redhat.com>
Date:   Tue, 01 Sep 2020 10:10:50 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Lai Jiangshan <jiangshanlai@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Lai Jiangshan <laijs@...ux.alibaba.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        kvm@...r.kernel.org
Subject: Re: [PATCH] kvm x86/mmu: use KVM_REQ_MMU_SYNC to sync when needed

Lai Jiangshan <jiangshanlai@...il.com> writes:

> On Mon, Aug 31, 2020 at 9:09 PM Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
>>
>> Lai Jiangshan <jiangshanlai@...il.com> writes:
>>
>> > Ping @Sean Christopherson
>> >
>>
>> Let's try 'Beetlejuice' instead :-)
>>
>> > On Mon, Aug 24, 2020 at 5:18 PM Lai Jiangshan <jiangshanlai@...il.com> wrote:
>> >>
>> >> From: Lai Jiangshan <laijs@...ux.alibaba.com>
>> >>
>> >> 8c8560b83390("KVM: x86/mmu: Use KVM_REQ_TLB_FLUSH_CURRENT for MMU specific flushes)
>> >> changed it without giving any reason in the changelog.
>> >>
>> >> In theory, the syncing is needed, and need to be fixed by reverting
>> >> this part of change.
>>
>> Even if the original commit is not wordy enough this is hardly
>> better.
>
> Hello,
> Thank you for reviewing it.
>
> I'm sorry that when I said "reverting this part of change",
> I meant "reverting this line of code". This line of code itself
> is quite clear that it is not related to the original commit
> according to its changelog.
>
>> Are you seeing a particular scenario when a change in current
>> vCPU's MMU requires flushing TLB entries for *other* contexts, ... (see
>> below)
>
> So I don't think the patch needs to explain this because the patch
> does not change/revert anything about it.
>
> Anyway, using a "revert" in the changelog is misleading, when it
> is not really reverting the whole targeted commit. I would
> remove this wording.
>
> For the change in my patch, when kvm_mmu_get_page() gets a
> page with unsync children, the host side pagetable is
> unsynchronized with the guest side pagedtable, and the
> guest might not issue a "flush" operation on it. It is
> all about the host's emulation of the pagetable. So the host
> has the responsibility to synchronize the pagetables.
>

Ah, I see now, so it seems Sean's commit has a stray change in it: the
intention was to change KVM_REQ_TLB_FLUSH -> KVM_REQ_TLB_FLUSH_CURRENT
so we don't unneedlesly flush other contexts but one of the hunks
changed KVM_REQ_MMU_SYNC instead. Syncronizing MMU roots can't be
replaced with a TLB flush, we need to revert back the change. This
sounds reasonable to me, please send out v2 with the updated
description. Thanks!

-- 
Vitaly