linux-kernel - Re: [PATCH v19 087/130] KVM: TDX: handle vcpu migration over logical processor

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Message-ID: <a552a48d-81b6-441a-88cf-63301f6968a2@intel.com>
Date: Tue, 16 Apr 2024 12:05:31 +1200
From: "Huang, Kai" <kai.huang@...el.com>
To: "Yamahata, Isaku" <isaku.yamahata@...el.com>, Sean Christopherson
	<seanjc@...gle.com>
CC: "Chatre, Reinette" <reinette.chatre@...el.com>, "kvm@...r.kernel.org"
	<kvm@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "isaku.yamahata@...il.com"
	<isaku.yamahata@...il.com>, Paolo Bonzini <pbonzini@...hat.com>, "Aktas,
 Erdem" <erdemaktas@...gle.com>, Sagi Shahar <sagis@...gle.com>, "Chen, Bo2"
	<chen.bo@...el.com>, "Yuan, Hang" <hang.yuan@...el.com>, "Zhang, Tina"
	<tina.zhang@...el.com>, "isaku.yamahata@...ux.intel.com"
	<isaku.yamahata@...ux.intel.com>
Subject: Re: [PATCH v19 087/130] KVM: TDX: handle vcpu migration over logical
 processor



On 16/04/2024 10:48 am, Yamahata, Isaku wrote:
> On Mon, Apr 15, 2024 at 06:49:35AM -0700,
> Sean Christopherson <seanjc@...gle.com> wrote:
> 
>> On Fri, Apr 12, 2024, Isaku Yamahata wrote:
>>> On Fri, Apr 12, 2024 at 03:46:05PM -0700,
>>> Sean Christopherson <seanjc@...gle.com> wrote:
>>>
>>>> On Fri, Apr 12, 2024, Isaku Yamahata wrote:
>>>>> On Fri, Apr 12, 2024 at 09:15:29AM -0700, Reinette Chatre <reinette.chatre@...el.com> wrote:
>>>>>>> +void tdx_mmu_release_hkid(struct kvm *kvm)
>>>>>>> +{
>>>>>>> +	while (__tdx_mmu_release_hkid(kvm) == -EBUSY)
>>>>>>> +		;
>>>>>>>   }
>>>>>>
>>>>>> As I understand, __tdx_mmu_release_hkid() returns -EBUSY
>>>>>> after TDH.VP.FLUSH has been sent for every vCPU followed by
>>>>>> TDH.MNG.VPFLUSHDONE, which returns TDX_FLUSHVP_NOT_DONE.
>>>>>>
>>>>>> Considering earlier comment that a retry of TDH.VP.FLUSH is not
>>>>>> needed, why is this while() loop here that sends the
>>>>>> TDH.VP.FLUSH again to all vCPUs instead of just a loop within
>>>>>> __tdx_mmu_release_hkid() to _just_ resend TDH.MNG.VPFLUSHDONE?
>>>>>>
>>>>>> Could it be possible for a vCPU to appear during this time, thus
>>>>>> be missed in one TDH.VP.FLUSH cycle, to require a new cycle of
>>>>>> TDH.VP.FLUSH?
>>>>>
>>>>> Yes. There is a race between closing KVM vCPU fd and MMU notifier release hook.
>>>>> When KVM vCPU fd is closed, vCPU context can be loaded again.
>>>>
>>>> But why is _loading_ a vCPU context problematic?
>>>
>>> It's nothing problematic.  It becomes a bit harder to understand why
>>> tdx_mmu_release_hkid() issues IPI on each loop.  I think it's reasonable
>>> to make the normal path easy and to complicate/penalize the destruction path.
>>> Probably I should've added comment on the function.
>>
>> By "problematic", I meant, why can that result in a "missed in one TDH.VP.FLUSH
>> cycle"?  AFAICT, loading a vCPU shouldn't cause that vCPU to be associated from
>> the TDX module's perspective, and thus shouldn't trigger TDX_FLUSHVP_NOT_DONE.
>>
>> I.e. looping should be unnecessary, no?
> 
> The loop is unnecessary with the current code.
> 
> The possible future optimization is to reduce destruction time of Secure-EPT
> somehow.  One possible option is to release HKID while vCPUs are still alive and
> destruct Secure-EPT with multiple vCPU context.  Because that's future
> optimization, we can ignore it at this phase.

I kinda lost here.

I thought in the current v19 code, you have already implemented this 
optimization?

Or is this optimization totally different from what we discussed in an 
earlier patch?

https://lore.kernel.org/lkml/8feaba8f8ef249950b629f3a8300ddfb4fbcf11c.camel@intel.com/