linux-kernel - Re: [RFC PATCH 09/12] KVM: TDX: Fold tdx_mem_page_record_premap

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Message-ID: <dd31f894-9f89-42da-bbcd-1be859ef1fcf@linux.intel.com>
Date: Fri, 29 Aug 2025 10:42:44 +0800
From: Binbin Wu <binbin.wu@...ux.intel.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
 "seanjc@...gle.com" <seanjc@...gle.com>, "Zhao, Yan Y" <yan.y.zhao@...el.com>
Cc: "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
 "pbonzini@...hat.com" <pbonzini@...hat.com>,
 "Annapurve, Vishal" <vannapurve@...gle.com>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "michael.roth@....com" <michael.roth@....com>,
 "Weiny, Ira" <ira.weiny@...el.com>
Subject: Re: [RFC PATCH 09/12] KVM: TDX: Fold tdx_mem_page_record_premap_cnt()
 into its sole caller



On 8/29/2025 2:52 AM, Edgecombe, Rick P wrote:
> On Thu, 2025-08-28 at 10:00 -0700, Sean Christopherson wrote:
>> On Thu, Aug 28, 2025, Yan Zhao wrote:
[...]
>>>
>>> 3. Unexpected zaps (such as kvm_zap_gfn_range()).
>> Side topic related to kvm_zap_gfn_range(), the KVM_BUG_ON() in vt_refresh_apicv_exec_ctrl()
>> is flawed.  If kvm_recalculate_apic_map() fails to allocate an optimized map, KVM
>> will mark APICv as inhibited, i.e. the associated WARN_ON_ONCE() is effectively
>> user-triggerable.
>>
>> Easiest thing would be to mark the vCPU as dead (though we obviously need
>> "KVM: Never clear KVM_REQ_VM_DEAD from a vCPU's requests" for that to be robust).
>>
>>
>>
> I'm going need to look up the related apic discussions from the base series and
> circle back.
There was an analysis about the inhibit reasons for TDX.
https://lore.kernel.org/lkml/e3a2e8fa-b496-4010-9a8c-bfeb131bc43b@linux.intel.com/

As Sean mentioned, if kvm_recalculate_apic_map() fails to allocate the memory
for optimized map, it will trigger the KVM_BUG_ON() in
vt_refresh_apicv_exec_ctrl(). And kvzalloc() failure should not be treated as
KVM bug.

As talking about user-triggerable, the kvzalloc() failure path could be
triggered by KVM_CREATE_VCPU and KVM_TDX_INIT_VCPU for TD. After
KVM_TDX_INIT_VCPU, the mapping is not allowed to be changed.

Sean's suggested code change looks good to me.