linux-kernel - Re: [PATCH] KVM/VMX: Check ept_pointer before flushing ept tlb

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Date:   Wed, 07 Nov 2018 11:49:05 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Tianyu Lan <ltykernel@...il.com>
Cc:     Lan Tianyu <Tianyu.Lan@...rosoft.com>, pbonzini@...hat.com,
        rkrcmar@...hat.com, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, hpa@...or.com, x86@...nel.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, michael.h.kelley@...rosoft.com,
        kys@...rosoft.com
Subject: Re: [PATCH] KVM/VMX: Check ept_pointer before flushing ept tlb

Tianyu Lan <ltykernel@...il.com> writes:

> Hi Vitaly:
> 	Thanks for your review.
>
> On 11/6/2018 11:50 PM, Vitaly Kuznetsov wrote:
>> ltykernel@...il.com writes:
>> 
>>> From: Lan Tianyu <Tianyu.Lan@...rosoft.com>
>>>
>>> This patch is to initialize ept_pointer to INVALID_PAGE and check it
>>> before flushing ept tlb. If ept_pointer is invalidated, bypass the flush
>>> request.
>>>
>> 
>> To be honest I fail to understand the reason behind the patch: instead
>> of doing one unneeded flush request with ept_pointer==0 (after vCPU is
>> initialized) we now do the check every time. Could you please elaborate
>> on why this is needed?
>
> The reason to introduce the check here is to avoid flushing ept tlb
> without valid ept table. When nested guest boots up and only BP is
> active, we should not do flush for APs and L1 hypervisor hasn't set
> valid EPT table for APs.

Yes, I understand that but I'm trying to avoid additional checks on
hotpath as during normal operation EPT pointer is always set.

Could we just initialize ept_pointers_match to something like
EPT_POINTERS_NOTSET and achive the same result?

-- 
Vitaly