linux-kernel - Re: [PATCH v2 5/8] KVM: TDX: Handle TDG.VP.VMCALL<MapGPA>

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Message-ID: <bcb80309-10ec-44e3-90db-259de6076183@linux.intel.com>
Date: Mon, 17 Feb 2025 11:41:48 +0800
From: Binbin Wu <binbin.wu@...ux.intel.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Chao Gao <chao.gao@...el.com>, Yan Zhao <yan.y.zhao@...el.com>,
 pbonzini@...hat.com, kvm@...r.kernel.org, rick.p.edgecombe@...el.com,
 kai.huang@...el.com, adrian.hunter@...el.com, reinette.chatre@...el.com,
 xiaoyao.li@...el.com, tony.lindgren@...el.com, isaku.yamahata@...el.com,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 5/8] KVM: TDX: Handle TDG.VP.VMCALL<MapGPA>



On 2/13/2025 11:17 PM, Sean Christopherson wrote:
> On Thu, Feb 13, 2025, Binbin Wu wrote:
>> On 2/13/2025 11:23 AM, Binbin Wu wrote:
>>> On 2/13/2025 2:56 AM, Sean Christopherson wrote:
>>>> On Wed, Feb 12, 2025, Binbin Wu wrote:
>>>>> On 2/12/2025 8:46 AM, Sean Christopherson wrote:
>>>>>> I am completely comfortable saying that KVM doesn't care about STI/SS shadows
>>>>>> outside of the HALTED case, and so unless I'm missing something, I think it makes
>>>>>> sense for tdx_protected_apic_has_interrupt() to not check RVI outside of the HALTED
>>>>>> case, because it's impossible to know if the interrupt is actually unmasked, and
>>>>>> statistically it's far, far more likely that it _is_ masked.
>>>>> OK. Will update tdx_protected_apic_has_interrupt() in "TDX interrupts" part.
>>>>> And use kvm_vcpu_has_events() to replace the open code in this patch.
>>>> Something to keep an eye on: kvm_vcpu_has_events() returns true if pv_unhalted
>>>> is set, and pv_unhalted is only cleared on transitions KVM_MP_STATE_RUNNABLE.
>>>> If the guest initiates a spurious wakeup, pv_unhalted could be left set in
>>>> perpetuity.
>>> Oh, yes.
>>> KVM_HC_KICK_CPU is allowed in TDX guests.
> And a clever guest can send a REMRD IPI.
>
>>> The change below looks good to me.
>>>
>>> One minor issue is when guest initiates a spurious wakeup, pv_unhalted is
>>> left set, then later when the guest want to halt the vcpu, in
>>> __kvm_emulate_halt(), since pv_unhalted is still set and the state will not
>>> transit to KVM_MP_STATE_HALTED.
>>> But I guess it's guests' responsibility to not initiate spurious wakeup,
>>> guests need to bear the fact that HLT could fail due to a previous
>>> spurious wakeup?
>> Just found a patch set for fixing the issue.
> FWIW, Jim's series doesn't address spurious wakeups per se, it just ensures
> pv_unhalted is cleared when transitioning to RUNNING.  If the vCPU is already
> RUNNING, __apic_accept_irq() will set pv_unhalted and nothing will clear it
> until the next transition to RUNNING (which implies at least an attempted
> transition away from RUNNING).
>
Indeed.

I am wondering why KVM doesn't clear pv_unhalted before the vcpu entering guest?
Is the additional memory access a concern or is there some other reason?