Re: [PATCH] KVM: X86: fix tlb_flush_guest()

[Lai Jiangshan <laijs@linux.alibaba.com>]

On 2021/5/28 08:26, Sean Christopherson wrote:
> On Fri, May 28, 2021, Lai Jiangshan wrote:
>>
>> On 2021/5/28 00:13, Sean Christopherson wrote:
>>> 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 problem is that record_steal_time() is called after KVM_REQ_MMU_RELOAD
> in vcpu_enter_guest() and so the reload request won't be recognized until the
> next VM-Exit.  It works for kvm_handle_invpcid() because vcpu_enter_guest() is
> guaranteed to run between the invcpid code and VM-Enter.

Kvm will recheck the request before VM-enter.
See kvm_vcpu_exit_request().
It won't lost any request, it just causes a additional iteration.

> 
>> The improvement code will go later, and will not be backported.
> 
> I would argue that introducing a potential performance regression is in itself a
> bug.  IMO, going straight to kvm_mmu_sync_roots() is not high risk.

I think we can introduce a kvm_mmu_sync_roots_all() which syncs current
root and handles prev_roots (mark need_sync or drop) as cleanup/preparation patch
and then fix the problem.

Do we need to backport the cleanup/preparation patch or just backport the way
with KVM_REQ_MMU_RELOAD?


> 
>> 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.
> 
> I like the idea, I just haven't gotten around to reviewing that patch yet.
> 
>>> 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).
> 
> Ya, I belated realized this :-)
> 
>>>           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.

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

>>
>>>    }
>>>