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

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Date:   Tue, 1 Sep 2020 09:29:13 +0800
From:   Lai Jiangshan <jiangshanlai@...il.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.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

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.

Thanks
Lai

> >>
> >> 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
>