From: Lai Jiangshan <laijs@linux.alibaba.com>
To: Sean Christopherson <seanjc@google.com>,
Paolo Bonzini <pbonzini@redhat.com>
Cc: "Lai Jiangshan" <jiangshanlai@gmail.com>,
linux-kernel@vger.kernel.org,
"Vitaly Kuznetsov" <vkuznets@redhat.com>,
"Wanpeng Li" <wanpengli@tencent.com>,
"Jim Mattson" <jmattson@google.com>,
"Joerg Roedel" <joro@8bytes.org>,
"Thomas Gleixner" <tglx@linutronix.de>,
"Ingo Molnar" <mingo@redhat.com>,
"Borislav Petkov" <bp@alien8.de>,
x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
"Radim Krčmář" <rkrcmar@redhat.com>,
kvm@vger.kernel.org, "Maxim Levitsky" <mlevitsk@redhat.com>
Subject: Re: [PATCH] KVM: X86: fix tlb_flush_guest()
Date: Fri, 28 May 2021 08:18:43 +0800 [thread overview]
Message-ID: <d96f8c11-19e6-2c2d-91ff-6a7a51fa1b9c@linux.alibaba.com> (raw)
In-Reply-To: <YK/FGYejaIu6EzSn@google.com>
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@linux.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.
> }
>
>
next prev parent reply other threads:[~2021-05-28 0:18 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-27 2:39 [PATCH] KVM: X86: fix tlb_flush_guest() Lai Jiangshan
2021-05-27 12:55 ` Paolo Bonzini
2021-05-27 16:13 ` Sean Christopherson
2021-05-27 16:14 ` Sean Christopherson
2021-05-27 19:28 ` Sean Christopherson
2021-05-28 1:13 ` Lai Jiangshan
2021-06-02 15:09 ` Sean Christopherson
2021-06-02 22:07 ` Sean Christopherson
2021-05-28 0:18 ` Lai Jiangshan [this message]
2021-05-28 0:26 ` Sean Christopherson
2021-05-28 1:29 ` Lai Jiangshan
2021-06-02 15:01 ` Sean Christopherson
2021-06-02 8:13 ` Lai Jiangshan
2021-05-29 22:12 ` Maxim Levitsky
2021-05-31 17:22 ` [PATCH V2] " Lai Jiangshan
2021-06-02 15:39 ` Sean Christopherson
2021-06-07 22:38 ` Maxim Levitsky
2021-06-08 0:03 ` Sean Christopherson
2021-06-08 14:01 ` Lai Jiangshan
2021-06-08 17:36 ` Paolo Bonzini
2021-06-08 21:31 ` Maxim Levitsky
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