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

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Message-ID: <875z8zx8qs.fsf@vitty.brq.redhat.com>
Date:   Mon, 31 Aug 2020 15:09:15 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Lai Jiangshan <jiangshanlai@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     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:

> 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. Are you seeing a particular scenario when a change in current
vCPU's MMU requires flushing TLB entries for *other* contexts, ... (see
below)

>>
>> Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com>
>> ---
>>  arch/x86/kvm/mmu/mmu.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
>> index 4e03841f053d..9a93de921f2b 100644
>> --- a/arch/x86/kvm/mmu/mmu.c
>> +++ b/arch/x86/kvm/mmu/mmu.c
>> @@ -2468,7 +2468,7 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu,
>>                 }
>>
>>                 if (sp->unsync_children)
>> -                       kvm_make_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu);
>> +                       kvm_make_request(KVM_REQ_MMU_SYNC, vcpu);

... in particular, why are you reverting only this hunk? Please elaborate.

>>
>>                 __clear_sp_write_flooding_count(sp);
>>
>> --
>> 2.19.1.6.gb485710b
>>
>

-- 
Vitaly