linux-kernel - Re: [PATCH] KVM: X86: fix tlb_flush

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Date:   Fri, 28 May 2021 08:18:43 +0800
From:   Lai Jiangshan <laijs@...ux.alibaba.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org,
        Vitaly Kuznetsov <vkuznets@...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@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        kvm@...r.kernel.org, Maxim Levitsky <mlevitsk@...hat.com>
Subject: Re: [PATCH] KVM: X86: fix tlb_flush_guest()



On 2021/5/28 00:13, Sean Christopherson wrote:
> +Maxim - A proper fix for this bug might fix your shadow paging + win10 boot
>           issue, this also affects the KVM_REQ_HV_TLB_FLUSH used for HyperV PV
> 	 flushing.
> 
> On Thu, May 27, 2021, Paolo Bonzini wrote:
>> On 27/05/21 04:39, Lai Jiangshan wrote:
>>> From: Lai Jiangshan <laijs@...ux.alibaba.com>
>>>
>>> For KVM_VCPU_FLUSH_TLB used in kvm_flush_tlb_multi(), the guest expects
>>> the hypervisor do the operation that equals to native_flush_tlb_global()
>>> or invpcid_flush_all() in the specified guest CPU.
>>>
>>> When TDP is enabled, there is no problem to just flush the hardware
>>> TLB of the specified guest CPU.
>>>
>>> But when using shadowpaging, the hypervisor should have to sync the
>>> shadow pagetable at first before flushing the hardware TLB so that
>>> it can truely emulate the operation of invpcid_flush_all() in guest.
>>
>> Can you explain why?
> 
> KVM's unsync logic hinges on guest TLB flushes.  For page permission modifications
> that require a TLB flush to take effect, e.g. making a writable page read-only,
> KVM waits until the guest explicitly does said flush to propagate the changes to
> the shadow page tables.  E.g. failure to sync PTEs could result in a read-only 4k
> page being writable when the guest expects it to be read-only.
> 
>> Also it is simpler to handle this in kvm_vcpu_flush_tlb_guest, using "if
>> (tdp_enabled).  This provides also a single, good place to add a comment
>> with the explanation of what invalid entries KVM_REQ_RELOAD is presenting.
> 
> Ya.
> 
> KVM_REQ_MMU_RELOAD is overkill, nuking the shadow page tables will completely
> offset the performance gains of the paravirtualized flush >
> And making a request won't work without revamping the order of request handling
> in vcpu_enter_guest(), e.g. KVM_REQ_MMU_RELOAD and KVM_REQ_MMU_SYNC are both
> serviced before KVM_REQ_STEAL_UPDATE.

Yes, it just fixes the said problem in the simplest way.
I copied KVM_REQ_MMU_RELOAD from kvm_handle_invpcid(INVPCID_TYPE_ALL_INCL_GLOBAL).
(If the guest is not preempted, it will call invpcid_flush_all() and will be handled
by this way)


The improvement code will go later, and will not be backported.
The proper way to flush guest is to use code in

https://lore.kernel.org/lkml/20210525213920.3340-1-jiangshanlai@gmail.com/
as:
+		kvm_mmu_sync_roots(vcpu);
+		kvm_make_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu); //or just call flush_current directly
+		for (i = 0; i < KVM_MMU_NUM_PREV_ROOTS; i++)
+			vcpu->arch.mmu->prev_roots[i].need_sync = true;

If need_sync patch is not accepted, we can just use kvm_mmu_sync_roots(vcpu)
to keep the current pagetable and use kvm_mmu_free_roots() to free all the other
roots in prev_roots.



> 
> Cleaning up and documenting the MMU related requests is on my todo list, but the
> immediate fix should be tiny and I can do my cleanups on top.
> 
> I believe the minimal fix is:
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 81ab3b8f22e5..b0072063f9bf 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3072,6 +3072,9 @@ static void kvm_vcpu_flush_tlb_all(struct kvm_vcpu *vcpu)
>   static void kvm_vcpu_flush_tlb_guest(struct kvm_vcpu *vcpu)
>   {
>          ++vcpu->stat.tlb_flush;
> +
> +       if (!tdp_enabled)
> +               kvm_mmu_sync_roots(vcpu);

it doesn't handle prev_roots which are also needed as
shown in kvm_handle_invpcid(INVPCID_TYPE_ALL_INCL_GLOBAL).

>          static_call(kvm_x86_tlb_flush_guest)(vcpu);

For tdp_enabled, I think it is better to use kvm_x86_tlb_flush_current()
to make it consistent with other shadowpage code.

>   }
>   
>